dt-bindings: add clock_pre extend enable panel property

This change adds the documentation for a new panel property
qcom,mdss-dsi-t-clk-pre-extend, that makes clock_pre extend
enable configurable.

Change-Id: I8daed96c9a31bfba0faeb63ca42c9221537fee80
Signed-off-by: Rajeev Nandan <rajeevny@codeaurora.org>