dts: rp1: DSI drivers to use newly defined MIPI byte source clocks.

Remove the "dummy" 72MHz fixed clock sources and associate DSI driver
with the new "variable" clock sources now defined in RP1 clocks.

Also add PLLSYS clock to DSI, which it will need as an alternative
clock source in those cases where DPI pixclock > DSI byteclock.

Signed-off-by: Nick Hollinghurst <nick.hollinghurst@raspberrypi.com>