soc/mediatek/mt8192: add rtc MT6359P driver

Add rtc MT6359P driver for rtc init and rtc eosc calibration. Refactor
mt8173 and mt8183 code by extracting common API. Move rtc_read and
rtc_write to each SoC folder, because mt8173 and mt8183 access rtc via
pmic wrapper, while mt8192 accesses it via pmif.

Reference datasheet:
  Document No: RH-D-2018-0101.

Signed-off-by: Yuchen Huang <yuchen.huang@mediatek.corp-partner.google.com>
Change-Id: I57d6738fdec148c7458b2024a0a8225415ca2f3e
Reviewed-on: https://review.coreboot.org/c/coreboot/+/46395
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Yu-Ping Wu <yupingso@google.com>

src/soc/mediatek/mt8192/bootblock.c

@@ -6,6 +6,7 @@
 #include <soc/mt6359p.h>
 #include <soc/pll.h>
 #include <soc/pmif.h>
+#include <soc/rtc.h>
 #include <soc/wdt.h>
 
 void bootblock_soc_init(void)
@@ -16,4 +17,5 @@ void bootblock_soc_init(void)
 	mtk_pmif_init();
 	mt6359p_init();
 	mt6315_init();
+	rtc_boot();
 }