dt-bindings: clock: add support for 7nm DSI PLL shadow clock

Add clock id for 7nm DSI PLL shadow clocks, which will be
used during dynamic dsi clock switch.

CRs-Fixed: 2449594
Change-Id: I6c8da87de10605a37caebfe16052b856ea71982c
Signed-off-by: Sandeep Panda <spanda@codeaurora.org>
Signed-off-by: Yujun Zhang <yujunzhang@codeaurora.org>