ARM: dts: dra7: add support for parallel NAND flash

DRA7xx platform has in-build GPMC and ELM h/w engines which can be used
for accessing externel NAND flash device. This patch:
- adds generic DT binding in dra7.dtsi for enabling GPMC and ELM h/w engines
- adds DT binding for Micron NAND Flash (MT29F2G16AADWP) present on dra7-evm
*Important*
	On DRA7 EVM, GPMC_WPN and NAND_BOOTn are controlled by DIP switch
	So following board settings are required for NAND device detection:
	SW5.9 (GPMC_WPN) = LOW
	SW5.1 (NAND_BOOTn) = HIGH

Signed-off-by: Minal Shah <minalkshah@gmail.com>
Signed-off-by: Pekon Gupta <pekon@ti.com>
Reviewed-by: Javier Martinez Canillas <javier@dowhile0.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>