dt-bindings: rockchip-dw-mshc: add optional rockchip, desired-num-phases

By default, dw_mmc-rockchip will execute tuning for each degree.
So we won't miss every point of the good sample windows. However,
probably the phases are linear inside the good sample window.
Actually we don't need to do tuning for each degree so that we could
save some time, for instance, probe the driver or resume from S3.

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

Documentation/devicetree/bindings/mmc/rockchip-dw-mshc.txt

@@ -32,6 +32,10 @@ Optional Properties:
   probing, low speeds or in case where all phases work at tuning time.
   If not specified 0 deg will be used.
 
+* rockchip,desired-num-phases: The desired number of times that the host
+  execute tuning when needed. If not specified, the host will do tuning
+  for 360 times, namely tuning for each degree.
+
 Example:
 
 	rkdwmmc0@12200000 {