clk: msm: mdss: add DT support for SSC frequency and PPM values

The SSC frequency and PPM values are currently hardcoded in the
DSI PLL driver. Add DT properties to specify the SSC frequency
and SSC PPM values for DSI SSC feature.

Change-Id: I0faed9f48694f7407c6855b067ffa4510d7e3fdd
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>