ARM: dts: am437x: Provide NAND ready pin

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

For NAND we don't use GPMC wait pin monitoring but
get the NAND Ready/Busy# status using GPIOlib.
GPMC driver provides the WAIT0 pin status over GPIOlib.

Read speed increases from 16516 KiB/ to 18813 KiB/s
and write speed was unchanged at 9941 KiB/s.

Measured using mtd_speedtest.ko.

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