mb/google/brya/var/agah: Enable DPTF oem_variables

Support oem_variables and change based on EC notify event.

BUG=b:238921409
TEST=emerge-draco coreboot
1. check ACPI object ODVX has oem_variable[0]=0
Name (ODVX, Package (0x06)
{
   0x00000000,
   0x00000000,
   0x00000000,
   0x00000000,
   0x00000000,
   0x00000000
}
2. check can get EC oem variable change notify in the kernel log

Change-Id: Ibd856563a43d73a3b1be09b3fbebca1b36b5eab1
Signed-off-by: Tony Huang <tony-huang@quanta.corp-partner.google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/66575
Reviewed-by: Sumeet R Pawnikar <sumeet.r.pawnikar@intel.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/google/brya/variants/agah/include/variant/ec.h

@@ -8,4 +8,7 @@
 /* Enable EC backed Keyboard Backlight in ACPI */
 #define EC_ENABLE_KEYBOARD_BACKLIGHT
 
+/* Enable DPTF OEM variable */
+#define DPTF_ENABLE_OEM_VARIABLES
+
 #endif

src/mainboard/google/brya/variants/agah/overridetree.cb

@@ -145,6 +145,9 @@ chip soc/intel/alderlake
 							.granularity = 1000,
 						}
 				}"
+				register "oem_data.oem_variables" = "{
+						[0] = 0x0
+				}"
 				## Charger Performance Control (Control, mA)
 				register "controls.charger_perf" = "{
 					[0] = { 255, 1700 },