clk: imx: Add common API for masking MMDC handshake

All i.MX6 SoCs need to mask unused MMDC channel's handshake
for low power modes, this patch provides common API for masking
the MMDC channel passed from caller.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>