ARM: OMAP2+: Add device-tree support for NOR flash

NOR flash is not currently supported when booting with device-tree
on OMAP2+ devices. Add support to detect and configure NOR devices
when booting with device-tree.

Add documentation for the TI GPMC NOR binding.

Signed-off-by: Jon Hunter <jon-hunter@ti.com>
Tested-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>