ARM: OMAP2+: gpmc: add DT bindings for OneNAND

This patch adds device tree bindings for OMAP OneNAND devices.
Tested on an OMAP3 3430 IGEPv2 board.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>