google/gru: Add SKU ID for Scarlet

Scarlet has SKU detection strapping pins now. This patch adds the code
to read them.

BUG=b:69373077

Change-Id: I8d889a845950923bc7b5be9b79792cf3c8b6b8ad
Signed-off-by: Julius Werner <jwerner@chromium.org>
Reviewed-on: https://review.coreboot.org/22697
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/google/gru/boardid.c

@@ -15,6 +15,7 @@
 
 #include <boardid.h>
 #include <console/console.h>
+#include <gpio.h>
 #include <stdlib.h>
 #include <soc/saradc.h>
 
@@ -75,3 +76,18 @@ uint32_t ram_code(void)
 {
 	return get_index(0, &cached_ram_id);
 }
+
+uint32_t sku_id(void)
+{
+	if (!IS_ENABLED(CONFIG_GRU_BASEBOARD_SCARLET))
+		return UNDEFINED_STRAPPING_ID;
+
+	static uint32_t sku_id = UNDEFINED_STRAPPING_ID;
+	if (sku_id != UNDEFINED_STRAPPING_ID)
+		return sku_id;
+
+	gpio_t pins[3] = {[2] = GPIO(3, D, 6), [1] = GPIO(3, D, 5),
+			  [0] = GPIO(3, D, 4)}; /* GPIO3_D4 is LSB */
+	sku_id = gpio_base2_value(pins, ARRAY_SIZE(pins));
+	return sku_id;
+}