mb/amd/mandolin: resize EC FMAP section to match EC firmware size

The EC firmware is 128k including its header, so there's no need to
reserve another 4k for the header.

TEST=Mandolin still boots.

Change-Id: Id3a7a087bf37461ca8ad3da9a809f13d7f0d570c
Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/42705
Reviewed-by: Raul Rangel <rrangel@chromium.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/amd/mandolin/mandolin.fmd

@@ -1,7 +1,7 @@
 FLASH@0xFF800000 0x800000 {
 	BIOS@0x0 {
-		EC@0x0 0x21000
+		EC@0x0 0x20000
-		RW_MRC_CACHE@0x21000 0x10000
+		RW_MRC_CACHE@0x20000 0x10000
 		FMAP 0x1000
 		COREBOOT(CBFS)
 	}