mtd: atmel_nand: Support variable RB_EDGE interrupts

The NFC controller used to accelerate the NAND transfers on SAMA5 chips
can use either RB_EDGE0 or RB_EDGE3 as its ready/busy interrupt bit.

Use the controller's compatible string to select the correct bit.

For the binding:
Acked-by: Rob Herring <robh@kernel.org>

Reviewed-by: Wenyou Yang <Wenyou.yang@atmel.com>
Tested-by: Wenyou Yang <wenyou.yang@atmel.com>
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Romain Izard <romain.izard.pro@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>