soc/amd: Ensure bank 0 is selected before accessing VBNV in CMOS

In AMD platforms, the bit 4 of CMOS's Register A (0x0a) is DV0 bank
selection (0 for Bank 0; 1 for Bank 1) [1]. Since the MC146818 driver
accesses VBNV via Bank 0, the bit must be cleared before we can save
VBNV to CMOS in verstage.

Usually there's no problem with that, because the Register A is
configured in cmos_init() in ramstage. However, if CMOS has lost power,
then in the first boot after that, the bit may contain arbitrary data in
verstage. If that bit happens to be 1, then CMOS writes in verstage will
fail.

To fix the problem, define vbnv_platform_init_cmos() to call
cmos_init(0), which will configure the Register A and therefore allow
saving VBNV to CMOS in verstage.

[1] 48751_16h_bkdg.pdf

BUG=b:346716300
TEST=CMOS writes succeeded in verstage after battery cutoff
BRANCH=skyrim

Change-Id: Idf167387b403be1977ebc08daa1f40646dd8c83f
Signed-off-by: Yu-Ping Wu <yupingso@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/83495
Reviewed-by: Matt DeVillier <matt.devillier@amd.corp-partner.google.com>
Reviewed-by: Karthik Ramasubramanian <kramasub@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/amd/common/vboot/vbnv_cmos.c

@@ -3,6 +3,12 @@
 #include <security/vboot/vbnv.h>
 #include <pc80/mc146818rtc.h>
 
+void vbnv_platform_init_cmos(void)
+{
+	/* The 0 argument tells cmos_init not to update CMOS unless it is invalid. */
+	cmos_init(0);
+}
+
 int vbnv_cmos_failed(void)
 {
 	/* If CMOS power has failed, the century will be set to 0xff */