pinctrl: mvebu: provide generic simple mmio-based implementation

Provide a generic simple mmio-based probe function and methods, which
pinctrl drivers can use to initialise the mvebu pinctrl subsystem.
Most mvebu pinctrl drivers can use this.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>