ARM: OMAP2+: Update GPMC and NAND DT binding documentation

Add additional details to the GPMC NAND documentation to clarify
what is needed to enable NAND DMA prefetch.

Signed-off-by: Franklin S Cooper Jr <fcooper@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>