Documentation: dt: fb: list mdss pll driver compatible strings

MDSS PLL driver can be used by several different interfaces,
determined by the compatible string. List all supported
interfaces for this driver in the documentation for
devicetree bindings.

Change-Id: Ia079eceefe57a1498d6a6fed0088f84a87b8ed6e
Signed-off-by: Casey Piper <cpiper@codeaurora.org>