disp: pll: add support for 7nm DSI PLL shadow clock

Add support for 7nm DSI PLL shadow clocks, which will be
used during dynamic dsi clock switch and dfps feature.

Change-Id: I870f961c7af4d404e61b45a4ad860ffb0e71ae7c
Signed-off-by: Sandeep Panda <spanda@codeaurora.org>
Signed-off-by: Yujun Zhang <yujunzhang@codeaurora.org>
Signed-off-by: Ray Zhang <rayz@codeaurora.org>