clk: rockchip: add basic infrastructure for clock branches

This adds infrastructure for registering clock branches. On Rockchip SoCs
most clock branches are a combination of mux,divider and gate components,
thus a composite clock is used when appropriate.

Clock branches are supposed to be declared in an array using the COMPOSITE*
or MUX, etc makros defined in the header and then registered using
rockchip_clk_register_branches.

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>