vendorcode/google/chromeos: Add mainboard hook before cr50 update

In order to allow the mainboard to configure the system before a
cr50 initiated update reset add a weak function that the mainboard
can override if necessary.

This will allow a board that would otherwise be configured to
stay off after an EC reset to instead power up after the reset and
not end up in a shut down state after a cr50 update.

BUG=b:121380403
TEST=update cr50 firmware on sarien and reboot

Change-Id: I11f9e8c9bfe810f69b4eaa2c633252c25004cbd0
Signed-off-by: Duncan Laurie <dlaurie@google.com>
Reviewed-on: https://review.coreboot.org/c/31057
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/vendorcode/google/chromeos/chromeos.h

@@ -33,6 +33,9 @@ static inline void mark_watchdog_tombstone(void) { return; }
 static inline void reboot_from_watchdog(void) { return; }
 #endif /* CONFIG_CHROMEOS */
 
+/* Defined as weak function in cr50_enable_update.c */
+void mainboard_cr50_update_reset(void);
+
 struct romstage_handoff;
 
 #include "gnvs.h"