mb/google/reef: Cache EC's SKU ID on Coral

Change-Id: I1925f51d63290b8d08366b622d5df3aab3a7484e
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/21737
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Sumeet R Pawnikar <sumeet.r.pawnikar@intel.com>

src/mainboard/google/reef/variants/coral/mainboard.c

@@ -26,8 +26,12 @@ enum {
 
 uint8_t variant_board_sku(void)
 {
-	// we know that the values are in 0..255
-	return google_chromeec_get_sku_id();
+	static int sku = -1;
+
+	if (sku == -1)
+		sku = google_chromeec_get_sku_id();
+
+	return sku;
 }
 
 void variant_nhlt_oem_overrides(const char **oem_id,