dt-bindings: clk: rp1: Add clocks representing MIPI DSI byteclock

Define two new RP1 clocks, representing the MIPI DSI byteclock
sources for the dividers used to generate MIPI[01] DPI pixel clocks.
(Previously they were represented by "fake" fixed clocks sources).

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