mb/google/herobrine: Update FMD file for multiple ROM sizes

The Piglin & Hoglin boards were built with a couple of different sizes
of ROM chips.  Despite this, the desire was to use just a single FMD
file.  The different sizes are already accounted for in Kconfig, so
add the Kconfig size here to be used.

Signed-off-by: Martin Roth <martin.roth@amd.corp-partner.google.com>
Change-Id: Ia75725b0c4d61e832c94160fa4cd455e89c60274
Reviewed-on: https://review.coreboot.org/c/coreboot/+/69939
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Reviewed-by: Shelley Chen <shchen@google.com>
Reviewed-by: Eric Lai <eric_lai@quanta.corp-partner.google.com>

src/mainboard/google/herobrine/chromeos.fmd

@@ -1,7 +1,12 @@
 ## SPDX-License-Identifier: GPL-2.0-only
 
 # TODO: update for Herobrine
+
+#if CONFIG_BOARD_GOOGLE_PIGLIN || CONFIG_BOARD_GOOGLE_HOGLIN
+FLASH@0x0 CONFIG_ROM_SIZE {
+#else
 FLASH@0x0 8M {
+#endif
 	WP_RO 4M {
 		RO_SECTION 0x3c4000 {
 			BOOTBLOCK 96K