ARM: dts: msm: add missing documentation for UFS PHY clocks

UFS PHY node requires clocks are mandatory property but it is not
documented hence this change adds the relevant documentation for it.

Change-Id: I07b04232386436e879d47c0684d4bbddcdc05929
Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>