mmc: dw_mmc-rockchip: parse rockchip, desired-num-phases from DT

Currently we unconditionally do tuning for each degree, which
costs 900ms for each boot and resume.

May someone argue that this is a question of accuracy VS time. But I
would say it's a trick of how we need to do decision for our boards.
If we don't care the time we spend at all, we could definitely do tuning
for each degree. But when we need to improve the user experience, for
instance, speed up resuming from S3, we should also have the right to
do that. This patch add parsing "rockchip,desired-num-phases", for folks
to specify the number of doing tuning. If not specified, 360 will be used
as before.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>