board: AM335x-ICEv2: Add DDR data

AM335x ICEv2 contains a 2Gbit(128Mx16) of DDR3 SDRAM(MT41J128M16JT-125),
capable of running at 400MHz. Adding this specific DDR configuration
details running at 400MHz.

Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>