pinctrl: spear: switch plgpio to irqchip helpers

This switches the SPEAr PLGPIO driver over to using the irqchip
helpers.

As part of this effort, also get rid of the strange irq_base
calculation and failure to use d->hwirq for obtaining a local
irqchip offset.

Cc: Viresh Kumar <viresh.linux@gmail.com>
Cc: Shiraz Hashim <shiraz.linux.kernel@gmail.com>
Cc: spear-devel@list.st.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>