omap2420-h4: Fix DRAM initialisation

The size of the DRAM for the omap2420-h4 board is getting setup in the
dram_init() function. However, for the current u-boot release this is
too late and needs to be done in dram_init_banksize(). Therefore, add
a dram_init_banksize() function for the omap2420-h4 board and setup the
DRAM size there.

Signed-off-by: Jon Hunter <jon-hunter@ti.com>