clk: rockchip: use clock ID for DMC (memory controller) on rk3288

The DMC clocks need to be turned off at runtime.  Use the newly
assigned clock IDs to 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>