gpio/mxc: move irq_domain_add_legacy call into gpio driver

Move irq_domain_add_legacy call from imx*-dt.c into gpio driver and
have the gpio driver adopt irqdomain support for both DT and non-DT
boot.

With all imx platform code converted from static gpio irq number
computation to use run-time gpio_to_irq call, we can now use
irq_alloc_descs and irqdomain support to dynamically get irq_base
and have the mapping between gpio and irq number available without
using virtual_irq_start and MXC_GPIO_IRQ_START.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Cc: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Dong Aisheng <dong.aisheng@linaro.org>