vc/google/chromeos: Refactor GNVS init

Move the support code for filling ChromeOS GNVS from
acpi/chromeos-gnvs.c to vc/google/chromeos/gnvs.c.

Change-Id: I7e92206561812eb3dc69739df49b6c3a93853858
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/50612
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Lance Zhao
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/include/acpi/acpi_gnvs.h

@@ -17,7 +17,6 @@ static inline void *acpi_get_gnvs(void) { return NULL; }
 static inline int acpi_reset_gnvs_for_wake(struct global_nvs **gnvs) { return -1; }
 #endif
 
-void gnvs_assign_chromeos(void *gnvs_section);
 void gnvs_set_ecfw_rw(void);
 
 /*