Documentation: msm: restructure coresight driver's binding

The required and optional properties for CoreSight drivers are not
described properly here. Restructure this device tree binding file
to make them more clear.

Change-Id: Ifc3400508f281e01d128fb6c782fa210bd880e10
Signed-off-by: Xiaogang Cui <xiaogang@codeaurora.org>