arm64: dts: ti: k3-j721e-common-proc-board: add assigned clks for DSS

The DSS related clock muxes are set via assigned-clocks in a way which
provides us:

VP0 - DisplayPort SST
VP1 - DPI0
VP2 - DSI
VP3 - DPI1

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Tero Kristo <t-kristo@ti.com>