mb/google/brox/var/caboc: Add PDC FW hash to hint romstage init

This CL follow Commit 2ba74b8c18 ("mb/google/brox: Hint romstage init
about upcoming reset") CB:84937 to add PDC FW hash in variant.c to hint
romstage init about upcoming reset.

BUG=b:445606386
TEST=Build Caboc BIOS image and boot to OS. Ensure that the hints are
provided correctly and 2 redundant resets are filtered out.

Change-Id: Ie029bf7faf991f520c42ffe22e610291ba98e078
Signed-off-by: Tony Huang <tony-huang@quanta.corp-partner.google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/89190
Reviewed-by: Karthik Ramasubramanian <kramasub@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/google/brox/variants/caboc/variant.c

@@ -21,5 +21,5 @@ const char *get_wifi_sar_cbfs_filename(void)
 
 const char *variant_get_auxfw_version_file(void)
 {
-	return NULL;
+	return "rts5453vb_GOOG0L01.hash";
 }