clk: rockchip: add binding ID for DMC (memory controller) clocks on rk3288

The DMC clocks need to be turned off at runtime, so we should have IDs
so we can export them.

Signed-off-by: Jeff Chen <cym@rock-chips.com>
[dianders: split into two patches; adjusted commit msg]
Signed-off-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>