pinctrl: single: support gpio request and free

Marvell's PXA/MMP silicon also match the behavior of pinctrl-single.
Each pin binds to one register. A lot of pins could be configured
as gpio.

GPIO range is defined as a child node of pinmux in .dtsi file. If those
pins are with the same gpio function configuration in the pinmux
register, they could be defined in the same GPIO range. For this new
child node, two properties are used.

reg = <the start of pinmux register in range, size of range>

pinctrl-single,gpio: <gpio base in range, the gpio function of the range
		in the pinmux register>

Signed-off-by: Haojian Zhuang <haojian.zhuang@gmail.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>