pinctrl: rockchip: generalize perpin driver-strength setting

The upcoming support for the RK3368 ARM64 SoC also supports perpin
drive strength settings (at different register positions), so generalize
the register and offset calculation to easily support this one too.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>