pinctrl/coh901: use irqdomain, allocate irqdescs

This switches the COH 901 pinctrl driver to allocate its GPIO
IRQs dynamically, and start to use a linear irqdomain to map
from the hardware IRQs.

This way we can cut away the complex allocation of IRQ numbers
from the <mach/irqs.h> file.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>