pinctrl/rockchip: separate struct rockchip_pin_bank to a head file

Separate struct rockchip_pin_bank to pinctrl-rockchip.h file, which will
be used by gpio-rockchip driver in the future.

Signed-off-by: Jianqun Xu <jay.xu@rock-chips.com>
Link: https://lore.kernel.org/r/20210816011948.1118959-3-jay.xu@rock-chips.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>