rockchip: rk3308-rock-s0: Fix SD-card boot on v1.1 hw revision

rockchip: rk3308-rock-s0: Fix SD-card boot on v1.1 hw revision

BootROM leave GPIO4_D6 configured as SDMMC_PWREN function and DW MMC
driver set PWREN high in dwmci_init().

However, HW revision prior to v1.2 must pull GPIO4_D6 low to access
sdmmc. For HW revision v1.2 the state of GPIO4_D6 has no impact.

Upstream Linux commit 26c100232b09 "arm64: dts: rockchip: Fix sdmmc
access on rk3308-rock-s0 v1.1 boards" fixed this issue by adding a
vcc_sd regulator.

Include the new vcc_sd regulator in SPL and enable required Kconfig
options to set GPIO4_D6 low to fix reading sdmmc on v1.1 hw revision.

Fixes: 25438c40a007 ("board: rockchip: Add Radxa ROCK S0")
Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>