mb/google/brox: enable WIFI_SAR

Add get_wifi_sar_cbfs_filename().  This function uses the FW_CONFIG
for WIFI to choose the right wifi_sar hex file.  Below is the file
mapping:

    wifi_sar_0.hex = wifi6
    wifi_sar_1.hex = wifi7

BUG=b:319302319
BRANCH=None
TEST=emerge-brox coreboot chromeos-bootimage

Change-Id: I212c80412141e7770a512bd8ccf4111963bab395
Signed-off-by: Shelley Chen <shchen@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/80085
Reviewed-by: Nick Vaccaro <nvaccaro@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/google/brox/Kconfig

@@ -53,6 +53,7 @@ config BOARD_GOOGLE_BASEBOARD_BROX
 
 config BOARD_GOOGLE_BROX
 	select BOARD_GOOGLE_BASEBOARD_BROX
+	select CHROMEOS_WIFI_SAR if CHROMEOS
 
 if BOARD_GOOGLE_BROX_COMMON
 

src/mainboard/google/brox/variants/brox/variant.c

@@ -1,9 +1,16 @@
 /* SPDX-License-Identifier: GPL-2.0-only */
 
+#include <assert.h>
+#include <baseboard/variants.h>
 #include <chip.h>
 #include <fw_config.h>
-#include <baseboard/variants.h>
+#include <sar.h>
 
 void variant_update_soc_chip_config(struct soc_intel_alderlake_config *config)
 {
 }
+
+const char *get_wifi_sar_cbfs_filename(void)
+{
+	return get_wifi_sar_fw_config_filename(FW_CONFIG_FIELD(WIFI));
+}