rockchip: kevin/gru: Slow memory down to 300 MHz

rockchip: kevin/gru: Slow memory down to 300 MHz

At the higher speeds stressapptest shows memory errors.  We don't want
to track down random problems due to simple memory corruption, so slow
memory back down to 300 MHz until someone figures out how to make it
faster without sacrificing reliability.

BRANCH=None
BUG=chrome-os-partner:54144
TEST=stressapptest -M 1024 -s 240

Change-Id: I2417f93f65b1491a028a63ce563ed7dd7831becc
Signed-off-by: Martin Roth <martinroth@chromium.org>
Original-Commit-Id:
Original-Change-Id: I02182b25e677e27e8541445938f9da9ae9553fa6
Original-Signed-off-by: Douglas Anderson <dianders@chromium.org>
Original-Reviewed-on: https://chromium-review.googlesource.com/350480
Original-Reviewed-by: Vadim Bendebury <vbendeb@chromium.org>
Reviewed-on: https://review.coreboot.org/15120
Tested-by: build bot (Jenkins)
Reviewed-by: Furquan Shaikh <furquan@google.com>