clk: clk-rp1: Add "varsrc" clocks to represent MIPI byte clocks

Add a new class of clocks to RP1 to represent clock sources whose
frequency changes at run-time as a side-effect of some other driver.
Specifically this is for the two MIPI DSI byte-clock sources.

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