clk: imx6sl: add mmdc ipg clocks

i.MX6SL has MMDC0 and MMDC1 ipg clock in CCM CCGR, add them into
clock tree for clock management.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>