AM3517:EMIF4: fix SDRAM size to 256Mb

With addition of "dram_init_banksize()" function from Heiko,Schocher
(commit ID: 561142af), the DRAM size
is getting configured wrongly to 512Mb (CS0 & CS1).
So fix it to 256Mb.

Signed-off-by: Vaibhav Hiremath <hvaibhav@ti.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>