brya/gimble: add `get_wifi_sar_cbfs_filename()` in variant.c

gimble only uses one WiFi SAR table, contained in a file named wifi_sar_0.hex

BUG=b:189068477
TEST=USE="project_gimble emerge-brya coreboot" and verify it builds
without error.

Signed-off-by: Mark Hsieh <mark_hsieh@wistron.corp-partner.google.com>
Change-Id: Ied030b79183cc6f962674260e7a82a7261b317ca
Reviewed-on: https://review.coreboot.org/c/coreboot/+/57616
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/google/brya/variants/gimble/Makefile.inc

@@ -4,3 +4,5 @@ bootblock-y += gpio.c
 ramstage-y += gpio.c
 
 ramstage-$(CONFIG_FW_CONFIG) += fw_config.c
+
+ramstage-y += variant.c

src/mainboard/google/brya/variants/gimble/variant.c

@@ -0,0 +1,8 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#include <sar.h>
+
+const char *get_wifi_sar_cbfs_filename(void)
+{
+	return "wifi_sar_0.hex";
+}