arm: vt8500: gpio: Devicetree support for arch-vt8500

Converted the existing arch-vt8500 gpio to a platform_device.
Added support for WM8505 and WM8650 GPIO controllers.
Replaced existing readl/writel calls with _relaxed variants.
Replaced existing unsigned variables with u32 to match register size.

Signed-off-by: Tony Prisk <linux@prisktech.co.nz>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>