arm/dts: OMAP2: Add support for OMAP2420H4 Board

Simple DTS file for OMAP2420H4 board adding memory information to allow
device-tree testing on an OMAP2420. OMAP2420H4 board has 64MB of RAM.

Verified that kernel boots with DT using a simple RAMDISK file-system on
OMAP2420H4.

Signed-off-by: Jon Hunter <jon-hunter@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>