In roc-rk3328-cc board, the signal voltage of sdmmc is supplied by the
vcc_sdio regulator, which is a mux between 1.8V and 3.3V, controlled by
a special output only gpio pin labeled "gpiomut_pmuio_iout",
corresponding bit 1 of the syscon GRF_SOC_CON10.

This special pin can now be reference as <&grf_gpio 0>, thanks to the
gpio-syscon driver, which makes writing regulator-gpio possible.

If the signal voltage changes, the io domain needs to change
correspondingly.

To use this feature, the following options are required in kernel config:
 - CONFIG_GPIO_SYSCON=y
 - CONFIG_POWER_AVS=y
 - CONFIG_ROCKCHIP_IODOMAIN=y
Signed-off-by: Levin Du <djw@t-chip.com.cn>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>