ARM: OMAP: gpmc: add DT bindings for GPMC timings and NAND

This patch adds basic DT bindings for OMAP GPMC.

The actual peripherals are instantiated from child nodes within the GPMC
node, and the only type of device that is currently supported is NAND.

Code was added to parse the generic GPMC timing parameters and some
documentation with examples on how to use them.

Successfully tested on an AM33xx board.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
[tony@atomide.com: updated to apply]
Signed-off-by: Tony Lindgren <tony@atomide.com>