clk: composite: allow read-only clocks

This allows readl-only composite clocks by making mux_ops->set_parent and
divider_ops->round_rate/set_rate optional.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
merge divs and muxes into the composite-clock definition
and rename everything to "branch"
convert rk3188 clocks to new scheme