dt-bindings: marvell,mmp2: Add clock id for the LCDC clock

The peripheral clock is required for access the the LCDC registers. It
is in fact separate from the "AXI clock" that is optionally used to
generate the pixel clock and as such requires a separate clock id.

Link: https://lists.freedesktop.org/archives/dri-devel/2019-January/203975.html


Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>