rockchip: pinctrl: rk3328: use gpio instead of sdmmc-pwren

SDMMC-PWREN is a pin to control voltage for SDMMC IO, it may
be high active or low active, the dwmmc driver always assume
the sdmmc-pwren as high active.

Kernel treat this pin as fixed regulator instead of a pin from
controller, and then it can set in dts file upon board schematic,
that's a good solution, we can also do this in u-boot.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Acked-by: Simon Glass <sjg@chromium.org>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>