Kirkwood: bugfix: DRAM size initialization

If start of any DRAM bank is greater than total DDR size, remaining DDR banks' start address & size were left un-initialized in dram_init function. This could break other functions who uses array 'gd->bd->bi_dram'. Kirkwood network driver is one example. This also stops Linux kernel from booting.

v2 - Set start address also to 0. Without this Linux kernel couldn't
     boot up

Signed-off-by: Tanmay Upadhyay <tanmay.upadhyay@einfochips.com>