mb/samsung/stumpy: Fix thermal configuration issues

Fix multiple critical thermal management problems while preserving the
quiet-at-idle design:

- Overlapping temperature thresholds causing fan oscillations
- CRITICAL_TEMPERATURE equal to Tj_max (no safety margin)
- Multiple fan levels trying to activate simultaneously

Issues fixed:

1. CRITICAL_TEMPERATURE: 100°C → 98°C
   - Was equal to Tj_max, leaving zero safety margin
   - System could reach absolute thermal limits before shutdown
   - 2°C margin allows clean shutdown before CPU thermal protection

2. Fix overlapping temperature thresholds (PRIMARY BUG)
   - Previous config had all fan levels overlapping:
     * FAN3: 48-55°C
     * FAN2: 52-64°C (started at 52°C, before FAN3 stopped at 55°C)
     * FAN1: 60-68°C (started at 60°C, before FAN2 stopped at 64°C)
     * FAN0: 66-78°C (started at 66°C, before FAN1 stopped at 68°C)
   - Multiple fan levels would try to activate simultaneously
   - Caused rapid fan speed oscillations and unpredictable behavior

   New configuration with proper discrete levels:
     * FAN3: 45-55°C
     * FAN2: 55-65°C (starts when FAN3 stops)
     * FAN1: 65-72°C (starts when FAN2 stops)
     * FAN0: 72-80°C (starts when FAN1 stops)

3. Increase PWM values for better cooling at each level:
   - FAN3: 0x40 → 0xA0
   - FAN2: 0x80 → 0xB0
   - FAN1: 0xb0 → 0xC0
   - Provides more effective cooling progression

Design philosophy:
- Keep FAN4_PWM = 0x00 (fan OFF at idle)
  * Chromebox designed as quiet desktop device
  * Passive cooling adequate below 55°C
  * Silent operation at idle/light loads

Configuration now follows best practices:
- Silent at idle (fan OFF until >45-55°C)
- No overlapping thresholds (discrete fan levels)
- 8-10°C hysteresis (prevents oscillation)
- Proper safety margin below Tj_max
- Progressive PWM values for smooth transitions

These changes fix the actual bugs (overlaps and safety margins) while
maintaining the intended quiet operation at idle.

TEST=build/boot stumpy, verify fan remains silent at idle, activates
smoothly when needed, no oscillations, proper cooling maintained under
load.

Change-Id: I284cbe34348345589564ae77828b9beee0b0d9c0
Signed-off-by: Matt DeVillier <matt.devillier@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/89847
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Sean Rhodes <sean@starlabs.systems>

src/mainboard/samsung/stumpy/thermal.h

@@ -3,33 +3,33 @@
 #ifndef STUMPY_THERMAL_H
 #define STUMPY_THERMAL_H
 
-/* Fan is OFF */
+/* Fan is OFF at idle (passive cooling) */
 #define FAN4_THRESHOLD_OFF	0
 #define FAN4_THRESHOLD_ON	0
 #define FAN4_PWM		0x00
 
 /* Fan is at LOW speed */
-#define FAN3_THRESHOLD_OFF	48
+#define FAN3_THRESHOLD_OFF	45
 #define FAN3_THRESHOLD_ON	55
-#define FAN3_PWM		0x40
+#define FAN3_PWM		0xA0
 
 /* Fan is at MEDIUM speed */
-#define FAN2_THRESHOLD_OFF	52
-#define FAN2_THRESHOLD_ON	64
-#define FAN2_PWM		0x80
+#define FAN2_THRESHOLD_OFF	55
+#define FAN2_THRESHOLD_ON	65
+#define FAN2_PWM		0xB0
 
 /* Fan is at HIGH speed */
-#define FAN1_THRESHOLD_OFF	60
-#define FAN1_THRESHOLD_ON	68
-#define FAN1_PWM		0xb0
+#define FAN1_THRESHOLD_OFF	65
+#define FAN1_THRESHOLD_ON	72
+#define FAN1_PWM		0xC0
 
 /* Fan is at FULL speed */
-#define FAN0_THRESHOLD_OFF	66
-#define FAN0_THRESHOLD_ON	78
+#define FAN0_THRESHOLD_OFF	72
+#define FAN0_THRESHOLD_ON	80
 #define FAN0_PWM		0xff
 
 /* Temperature which OS will shutdown at */
-#define CRITICAL_TEMPERATURE	100
+#define CRITICAL_TEMPERATURE	98
 
 /* Temperature which OS will throttle CPU */
 #define PASSIVE_TEMPERATURE	90