mb/google/fatcat/var/kinmen: Support SAR table selection via FW_CONFIG

Add wifi SAR table support for kinmen. Bit 2 in the CBI/FW_CONFIG
field is used to determine which SAR table file to load at runtime.

BUG=b:424628935
TEST= check coreboot log:
before:

[WARN ] CBFS: 'wifi_sar_defaults.hex' not found.
[ERROR] Failed to get the wifi_sar_defaults.hex file size!
[ERROR] failed getting SAR limits!

after:

[INFO ] Use wifi_sar_0.hex.
[INFO ] CBFS: Found 'wifi_sar_0.hex' @0x7072c0 size 0x116 in mcache @0x73abdd28

Change-Id: I63e44ba0a4094264165cc9bd7e41c4bd2094c242
Signed-off-by: Ivy Jian <ivy.jian@quanta.corp-partner.google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/88458
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Kapil Porwal <kapilporwal@google.com>
Reviewed-by: Subrata Banik <subratabanik@google.com>

src/mainboard/google/fatcat/variants/kinmen/Makefile.mk

@@ -4,3 +4,4 @@ bootblock-y += gpio.c
 romstage-y += gpio.c
 romstage-y += memory.c
 ramstage-y += gpio.c
+ramstage-$(CONFIG_FW_CONFIG) += variant.c

src/mainboard/google/fatcat/variants/kinmen/overridetree.cb

@@ -1,4 +1,7 @@
 fw_config
+	field WIFI 2 2
+		option WIFI_CNVI_7		0
+	end
 	field AUDIO 3 5
 		option AUDIO_UNKNOWN		0
 		option AUDIO_ALC721_SNDW	1

src/mainboard/google/fatcat/variants/kinmen/variant.c

@@ -0,0 +1,9 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#include <fw_config.h>
+#include <sar.h>
+
+const char *get_wifi_sar_cbfs_filename(void)
+{
+	return get_wifi_sar_fw_config_filename(FW_CONFIG_FIELD(WIFI));
+}