drivers/amd/agesa: Don't save regular boot MTRR to flash

Save the regular boot MTRRs that are restored on the S3 path during
the CPU init in cbmem instead of storing them to the SPI flash.

This was probably done because historically this code run with late
cbmem init (in ramstage).

TESTED on pcengines/apu1 and lenovo/g505s: S3 resume works fine.

Change-Id: Ia58e7cd1afb785ba0c379ba75ef6090b56cb9dc6
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/44294
Reviewed-by: Mike Banon <mikebdp2@gmail.com>
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-by: Michał Żygowski <michal.zygowski@3mdeb.com>

src/cpu/amd/agesa/Kconfig

@@ -50,7 +50,7 @@ config S3_DATA_POS
 
 config S3_DATA_SIZE
 	int
-	default 8192
+	default 4096
 
 endif # CPU_AMD_AGESA