ARM: OMAP2+: Add GPMC DT support for Ethernet child nodes

Besides being used to interface with external memory devices,
the General-Purpose Memory Controller can be used to connect
Pseudo-SRAM devices such as ethernet controllers to OMAP2+
processors using the TI GPMC as a data bus.

This patch allows an ethernet chip to be defined as an GPMC
child device node.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Jon Hunter <jon-hunter@ti.com>