mb/amd/birman_plus: Add SMMSTORE to FMAP

Add the SMMSTORE region to the default FMAP to allow
building for EDK2 as payload.

TEST: Still boots on AMD/Birman+

Change-Id: I661fcc55bf30aa6f1f3cc8a57e6d0eaf2fed4621
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/86177
Reviewed-by: Maximilian Brune <maximilian.brune@9elements.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/amd/birman_plus/board_glinda.fmd

@@ -3,6 +3,7 @@ FLASH@0xFF000000 16M {
 		EC_SIG 4K
 		FMAP 4K
 		COREBOOT(CBFS)
+		SMMSTORE(PRESERVE) 256K
 		EC_BODY@15872K 256K
 		RW_MRC_CACHE 120K
 	}