gpio: mxs: need to check return value of irq_alloc_generic_chip

Need to check return value of irq_alloc_generic_chip, because
it may return NULL.
1. Change mxs_gpio_init_gc return type from void to int.
2. Add a new lable out_irqdomain_remove to remove the irq domain
   when mxc_gpio_init_gc fail.

Signed-off-by: Peng Fan <van.freenix@gmail.com>
Cc: Alexandre Courbot <gnurou@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>