dt-bindings: memory-controllers: Add binding for jz4780-nemc

Add device tree bindings for the NAND/External Memory Controller (NEMC)
on Ingenic JZ4780

Signed-off-by: Alex Smith <alex@alex-smith.me.uk>
Signed-off-by: Zubair Lutfullah Kakakhel <Zubair.Kakakhel@imgtec.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>