mb/google/skywalker: Initialize flash controller in bootblock

Initialize SPI NOR Flash Controller (SNFC) in the bootblock to
enable CBFS and execute other stages such as verstage, romstage,
ramstage, etc.

BUG=b:379008996
BRANCH=none
TEST=Read NOR flash data successfully.

Signed-off-by: Noah Shen <noah.shen@mediatek.corp-partner.google.com>
Change-Id: Icf4af32dd9d8c704fd7246adda94dfa3350bb672
Reviewed-on: https://review.coreboot.org/c/coreboot/+/86998
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Yidi Lin <yidilin@google.com>
Reviewed-by: Yu-Ping Wu <yupingso@google.com>

src/mainboard/google/skywalker/Kconfig

@@ -27,4 +27,8 @@ config MAINBOARD_DIR
 config MAINBOARD_PART_NUMBER
 	string
 	default "Skywalker" if BOARD_GOOGLE_SKYWALKER
+
+config BOOT_DEVICE_SPI_FLASH_BUS
+	int
+	default 7
 endif

src/mainboard/google/skywalker/bootblock.c

@@ -1,8 +1,9 @@
 /* SPDX-License-Identifier: GPL-2.0-only OR MIT */
 
 #include <bootblock_common.h>
+#include <soc/spi.h>
 
 void bootblock_mainboard_init(void)
 {
-	/* TODO: add mainboard init for bootblock */
+	mtk_snfc_init();
 }