ARM: dts: OMAP2420: Correct H4 board memory size

The system memory node for the OMAP2420 H4 was incorrectly defined as
start address followed by end address, instead of start address and
size. No noticable side-effects were observed but fix this for
correctness.

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