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.