mb/system76/ptl: Increase size of SMMSTORE to 512KB

Ref: commit d32a372 ("drivers/smmstore: Increase default size of store to 512KB")
Signed-off-by: Tim Crawford <tcrawford@system76.com>

Author: crawfxrd

src/mainboard/system76/ptl/variants/lemp14/board.fmd

@@ -3,7 +3,7 @@ FLASH 32M {
 	SI_ME 9176K
 	SI_BIOS@16M 16M {
 		RW_MRC_CACHE 64K
-		SMMSTORE(PRESERVE) 256K
+		SMMSTORE(PRESERVE) 512K
 		WP_RO {
 			FMAP 4K
 			COREBOOT(CBFS)