mainboard/google/reef: add variant API for board_id and gpio

Provide APIs for the board_id() and gpio table functionality.
Default and weak implementations are provided from the baseboard.

BUG=chrome-os-partner:56677

Change-Id: I02d8deb7f60f8c4842916a9d35f51d8af74b1da4
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/16419
Tested-by: build bot (Jenkins)
Reviewed-by: Furquan Shaikh <furquan@google.com>

src/mainboard/google/reef/mainboard.c

@@ -14,6 +14,7 @@
  */
 
 #include <arch/acpi.h>
+#include <baseboard/variants.h>
 #include <boardid.h>
 #include <console/console.h>
 #include <device/device.h>
@@ -27,11 +28,14 @@
 static void mainboard_init(void *chip_info)
 {
 	int boardid;
+	const struct pad_config *pads;
+	size_t num;
 
 	boardid = board_id();
 	printk(BIOS_INFO, "Board ID: %d\n", boardid);
 
-	gpio_configure_pads(gpio_table, ARRAY_SIZE(gpio_table));
+	pads = variant_gpio_table(&num);
+	gpio_configure_pads(pads, num);
 
 	mainboard_ec_init();
 }