clk: rockchip: add bindings for the mmc clocks

These clocks represent the physical clocks (including phases) and they will
later be used for clock phase tuning.

Suggested-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Alexandru M Stan <amstan@chromium.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>