Add initial device-tree support for OMAP4 SoC.

This is based on the original panda board patch done by Manju:
http://permalink.gmane.org/gmane.linux.ports.arm.omap/60393

Add the generic GIC interrupt-controller from ARM.

Add an empty "soc" node to contain non memory mapped IPs
(DSP, MPU, IPU...).

Note: Since reg, irq and dma are provided by hwmod for the
moment, these attributes will not be present at all in DTS
to highlight the gap. They will be added as soon as dma bindings
will be there and drivers will be adapted.
Signed-off-by: Benoit Cousson <b-cousson@ti.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Cc: G, Manjunath Kondaiah <manjugk@ti.com>