rockchip: rk3399: improve sdram driver

rockchip: rk3399: improve sdram driver

improve rk3399 sdram drvier, so we can support DDR3,
and check the cs training result, so we make sdram
work more stable.

BRANCH=none
BUG=chrome-os-partner:51537
TEST=boot from kevin, do memtester in kernel and pass

Change-Id: I508bf26fb8163bab2d725a91ead929df585e04a7
Signed-off-by: Martin Roth <martinroth@google.com>
Original-Commit-Id: 4d83a87c459167145b7260f9af5c0380caddc056
Original-Change-Id: Id385f1343804a829b6589f89f4cfbb6565d41417
Original-Signed-off-by: Lin Huang <hl@rock-chips.com>
Original-Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
Original-Reviewed-on: https://chromium-review.googlesource.com/342664
Original-Commit-Ready: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-on: https://review.coreboot.org/14849
Tested-by: build bot (Jenkins)
Reviewed-by: Vadim Bendebury <vbendeb@chromium.org>