google/chromeos: Support AP watchdog flag from Chrome EC

After ChromiumOS CL:1293132 and CL:1295890, Chrome EC can store the flag
telling if the last reboot was triggered by AP watchdog for some boards
(e.g., Kukui).

This CL adds a new function google_chromeec_get_ap_watchdog_flag(),
which reads the AP watchdog flag from Chrome EC, and updates the tables
of reset causes and reset flags.

A new Kconfig option CHROMEOS_USE_EC_WATCHDOG_FLAG is added for
elog_handle_watchdog_tombstone() to determine if watchdog reset was
triggered by the AP watchdog flag from EC instead of the tombstone in
AP.

BUG=b:109900671,b:118654976
BRANCH=none
TEST=test with https://review.coreboot.org/c/coreboot/+/31843

Change-Id: I7a970666a8c6da32ac1c6af8280e808fe7fc106d
Signed-off-by: You-Cheng Syu <youcheng@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/31834
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Hung-Te Lin <hungte@chromium.org>

src/ec/google/chromeec/ec.h

@@ -64,6 +64,7 @@ int google_chromeec_kbbacklight(int percent);
 void google_chromeec_post(u8 postcode);
 int google_chromeec_vbnv_context(int is_read, uint8_t *data, int len);
 uint8_t google_chromeec_get_switches(void);
+bool google_chromeec_get_ap_watchdog_flag(void);
 
 /* Temporary secure storage commands */
 int google_chromeec_vstore_supported(void);