pinctrl: gpio: vt8500: Add pincontrol driver for arch-vt8500

This patch adds support for the GPIO/pinmux controller found on the VIA
VT8500 and Wondermedia WM8xxx-series SoCs.

Each pin within the controller is capable of operating as a GPIO or as
an alternate function. The pins are numbered according to their control
bank/bit so that if new pins are added, the existing numbering is maintained.

All currently supported SoCs are included: VT8500, WM8505, WM8650, WM8750 and
WM8850.

Signed-off-by: Tony Prisk <linux@prisktech.co.nz>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>