msm: sde: add support for optional rotator clocks

Some clocks are optional on different hardware version. Add
support to specify if certain clock is mandatory or not, and
allow clock to be optional for smmu node.

CRs-Fixed: 2009709
Change-Id: If5cd8edc35cb930cad1dfd0432f543f389e6fef4
Signed-off-by: Alan Kwong <akwong@codeaurora.org>