ARM: dts: am335x: 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 7869 KiB/ to 8875 KiB/s
and write speed was unchanged at 5100 KiB/s.

Measured using mtd_speedtest.ko on am335x-evm.

Cc: Teresa Remmet <t.remmet@phytec.de>
Cc: Ilya Ledvich <ilya@compulab.co.il>
Cc: Yegor Yefremov <yegorslists@googlemail.com>
Cc: Rostislav Lisovy <lisovy@gmail.com>
Cc: Enric Balletbo i Serra <eballetbo@iseebcn.com>
Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>