pinctrl: SPEAr: Add plgpio driver

Most of SPEAr SoCs, which support pinctrl, can configure & use
pads as gpio. This patch adds plgpio driver for configuring
these pads as gpio.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>