drivers/pc80/rtc/option.c: Reset only CMOS range covered by checksum

Proposed in the comment of commit 29030d0f3d
("drivers/pc80/rtc/option.c: Stop resetting CMOS during s3 resume"),
during sanitize_cmos(), only reset CMOS range covered by checksum and
the checksum itself from the file cmos.default in CBFS, in order to
prevent other runtime data in CMOS (e.g. the DRAM training data on
GM45 platforms for s3 resume) being erased.

Tested: cherry-pick this commit before commit 44a48ce7a4 ("Kconfig:
	Bring HEAP_SIZE to a common, large value"), which is already
	before my commit 29030d0f3d , Thinkpad X200 with
	CONFIG(STATIC_OPTION_TABLE) can resume from s3 again,
	indicating that DRAM training data are no longer erased.

Signed-off-by: Bill XIE <persmule@hardenedlinux.org>
Co-authored-by: Jonathon Hall <jonathon.hall@puri.sm>
Change-Id: I872bf5f41422bc3424cd8631e932aaae2ae82f7a
Reviewed-on: https://review.coreboot.org/c/coreboot/+/78906
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Jonathon Hall <jonathon.hall@puri.sm>

src/drivers/pc80/rtc/option.c

@@ -213,9 +213,12 @@ void sanitize_cmos(void)
 			return;
 
 		u8 control_state = cmos_disable_rtc();
-		/* Max length of 256 spans bank 0 and bank 1 */
-		for (i = 14; i < MIN(256, length); i++)
+		/* Copy checked range and the checksum from the default */
+		for (i = LB_CKS_RANGE_START; i < MIN(LB_CKS_RANGE_END + 1, length); i++)
 			cmos_write_inner(cmos_default[i], i);
+		/* CMOS checksum takes 2 bytes */
+		cmos_write_inner(cmos_default[LB_CKS_LOC], LB_CKS_LOC);
+		cmos_write_inner(cmos_default[LB_CKS_LOC + 1], LB_CKS_LOC + 1);
 		cmos_restore_rtc(control_state);
 	}
 }