memory: omap-gpmc: Support WAIT pin edge interrupts

OMAPs can have 2 to 4 WAITPINs that can be used as edge triggered
interrupts if not used for memory wait state insertion.

Support these interrupts via the gpmc IRQ domain.

The gpmc IRQ domain interrupt map is:

0 - NAND_fifoevent
1 - NAND_termcount
2 - GPMC_WAIT0 edge
3 - GPMC_WAIT1 edge, and so on

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