gpio/mxs: adopt irq_domain support for mxs gpio driver

Remove irq_domain_add_legacy call from mach-mxs.c and have the gpio
driver adopt irqdomain support, so that we can have the mapping
between gpio and irq number available without using virtual_irq_start
and MXS_GPIO_IRQ_START.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>