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>
Acked-By: Max Schwarz <max.schwarz@online.de>
Tested-By: Max Schwarz <max.schwarz@online.de>
Signed-off-by: Mike Turquette <mturquette@linaro.org>