tegra124: sdram: flush cal_auto_config register write

To ensure the writing of cal_auto_config register reaches the hardware
before we start waiting.

BUG=chrome-os-partner:48935
TEST=run reboots test
BRANCH=none

Change-Id: I8453ee80ae673b2b01eb39dbd47e4c8ef0c77822
Signed-off-by: Yen Lin <yelin@nvidia.com>
Reviewed-on: https://chromium-review.googlesource.com/334417
Reviewed-by: Julius Werner <jwerner@chromium.org>
Reviewed-by: David Hendricks <dhendrix@chromium.org>