ram: rk3328: only do data traning for cs0

No need to do twice data training for rk3328 ddr sdram, we re-use the
setting for both channel. And adjust the sdram_init properly for correct
init flow.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Signed-off-by: YouMin Chen <cym@rock-chips.com>