ARM: dts: dra7x-evm: Provide NAND ready pin

On these boards NAND ready pin status is avilable over
GPMC_WAIT0 pin.

Read speed increases from 13768 KiB/ to 17246 KiB/s.
Write speed was unchanged at 7123 KiB/s.
Measured using mtd_speedtest.ko.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>