mb/google/rex/var/karis: add hook for WiFi SAR table

WiFi SAR table for karis will be place into the CBFS later on and
as a result adding the hook in coreboot to make use of the SAR
table once the table is available.

BUG=b:290689824
TEST=emerge-rex coreboot

Change-Id: Ic989024ab9eb0fc439fc701c335a85986c4cfec5
Signed-off-by: YH Lin <yueherngl@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/78260
Reviewed-by: Eric Lai <ericllai@google.com>
Reviewed-by: Subrata Banik <subratabanik@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/google/rex/Kconfig

@@ -75,6 +75,7 @@ config BOARD_GOOGLE_MODEL_SCREEBO
 config BOARD_GOOGLE_MODEL_KARIS
 	def_bool n
 	select BOARD_GOOGLE_BASEBOARD_REX
+	select CHROMEOS_WIFI_SAR if CHROMEOS
 
 config BOARD_GOOGLE_BASEBOARD_OVIS
 	def_bool n

src/mainboard/google/rex/variants/karis/Makefile.inc

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

src/mainboard/google/rex/variants/karis/variant.c

@@ -0,0 +1,11 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#include <baseboard/variants.h>
+#include <chip.h>
+#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_TYPE));
+}