memory: omap-gpmc: Implement IRQ domain for NAND IRQs

GPMC provides 2 interrupts for NAND use. i.e. fifoevent and termcount.
Use IRQ domain for this. NAND device tree node can then
get the necessary interrupts by using gpmc as the interrupt parent.

Legacy boot uses gpmc_get_client_irq to get the
NAND interrupts from the GPMC IRQ domain.
Get rid of custom bitmasks and use IRQ domain for that
as well.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Tony Lindgren <tony@atomide.com>