soc/mediatek/mt8192: add pmic MT6315 driver

MT6315 is a buck converter for Mediatek MT8192 platform.
Reference datasheet: MT6315 datasheet v1.4.2.pdf, RH-D-2019-0616.

BUG=b:155253454
BRANCH=none
TEST=boot asurada correctly

Signed-off-by: Hsin-Hsiung Wang <hsin-hsiung.wang@mediatek.com>
Change-Id: I6b47473ee5d56a197bd21d4ab9b539d9663b6636
Reviewed-on: https://review.coreboot.org/c/coreboot/+/45400
Reviewed-by: Hung-Te Lin <hungte@chromium.org>
Reviewed-by: Yu-Ping Wu <yupingso@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/mediatek/mt8192/bootblock.c

@@ -2,6 +2,7 @@
 
 #include <bootblock_common.h>
 #include <soc/mmu_operations.h>
+#include <soc/mt6315.h>
 #include <soc/mt6359p.h>
 #include <soc/pll.h>
 #include <soc/pmif.h>
@@ -14,4 +15,5 @@ void bootblock_soc_init(void)
 	mt_pll_init();
 	mtk_pmif_init();
 	mt6359p_init();
+	mt6315_init();
 }