mb/amd/mandolin: add missing Kconfig type to CBFS_SIZE option

Change-Id: Ia4226537d17bb3732086980fb4e8de6bd1eaedbb
Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/42780
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/mainboard/amd/mandolin/Kconfig

@@ -27,6 +27,7 @@ config AMD_LPC_DEBUG_CARD
 	  Note that Kconfig does not currently enforce this restriction.
 
 config CBFS_SIZE
+	hex
 	default 0x7cf000 # Maximum size for the Mandolin FMAP
 
 config MAINBOARD_DIR