gpio: syscon: rockchip: add GRF GPIO support for rk3328

In Rockchip RK3328, the output only GPIO_MUTE pin, originally for codec
mute control, can also be used for general purpose. It is manipulated by
the GRF_SOC_CON10 register in GRF. Aside from the GPIO_MUTE pin, the HDMI
pins can also be set in the same way.

Currently this GRF GPIO controller only supports the mute pin. If needed
in the future, the HDMI pins support can also be added.

Signed-off-by: Levin Du <djw@t-chip.com.cn>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>