ec/google/chromeec: Add helper for critical battery state detection

Implement google_chromeec_is_critically_low_on_battery() to check if
the system is at risk of an imminent power-off.

This function returns true only if the battery is below the critical
threshold and no charger is detected. Combining these checks into a
single helper ensures that firmware notifications (like a red
lightbar) or emergency power-down logic do not trigger while the
device is successfully connected to AC power.

- Implement google_chromeec_is_critically_low_on_battery in ec.c.
- Export the function in ec.h for use in romstage/ramstage.

TEST=Verified on Fatcat & Bluey that the function correctly identifies
the low-power state and suppresses warnings when a charger is plugged
in.

Change-Id: I9f0f268d6660d913f989a9deffa24ab1f585b508
Signed-off-by: Subrata Banik <subratabanik@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/90963
Reviewed-by: Caveh Jalali <caveh@chromium.org>
Reviewed-by: Kapil Porwal <kapilporwal@google.com>
Reviewed-by: Jayvik Desai <jayvik@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/ec/google/chromeec/ec.c

@@ -1981,3 +1981,14 @@ int google_chromeec_set_lightbar_rgb(unsigned int led, int red, int green,
 
 	return google_chromeec_command(&cmd);
 }
+
+/*
+ * Check if the battery is critically low and not currently charging.
+ *
+ * Return true if battery is below threshold and AC is not present.
+ */
+bool google_chromeec_is_critically_low_on_battery(void)
+{
+	return google_chromeec_is_below_critical_threshold() &&
+	       !google_chromeec_is_charger_present();
+}

src/ec/google/chromeec/ec.h

@@ -562,4 +562,11 @@ int google_chromeec_read_batt_state_of_charge(uint32_t *state);
 int google_chromeec_set_lightbar_rgb(unsigned int led, int red, int green,
 			 int blue);
 
+/*
+ * Check if the battery is critically low and AC is not present.
+ *
+ * Return true if battery is below threshold and AC is not present.
+ */
+bool google_chromeec_is_critically_low_on_battery(void);
+
 #endif /* _EC_GOOGLE_CHROMEEC_EC_H */