mb/ocp/deltalake: add RW_MRC_CACHE flash region

Add RW_MRC_CACHE flash region to hold MRC cache data.

With memory training skipped for subsequent reboots, the boot
time is reduced by 8 minutes on OCP Delta Lake server, when
FSP verbose logging is turned on.

Signed-off-by: Jonathan Zhang <jonzhang@fb.com>
Change-Id: I27ed00100e1ea9e29b0e71ea5a8397cd550e193a
Reviewed-on: https://review.coreboot.org/c/coreboot/+/42025
Reviewed-by: Philipp Deppenwiese <zaolin.daisuki@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/ocp/deltalake/board.fmd

@@ -6,6 +6,7 @@ FLASH 64M {
 	}
 	SI_BIOS@0x3000000 0x1000000 {
 		FMAP@0x0 0x800
-		COREBOOT(CBFS)@0x800 0xfff800
+		RW_MRC_CACHE@0x1000 0x10000
+		COREBOOT(CBFS)@0x11000
 	}
 }