dt-bindings: Add frame threshold property for dsi controller

Change adds frame threshold time property in microseconds
to the controller. This specifies the idle time of controller
in command mode operation and is used to calculate the dsi
clocks if there is no preferred clock for the panel.

Change-Id: I24519602c48bffa8b13fcb49f7d3648b09733708
Signed-off-by: Vara Reddy <varar@codeaurora.org>
Signed-off-by: Ray Zhang <rayz@codeaurora.org>
Signed-off-by: Bruce Hoo <bingchua@codeaurora.org>