clk: imx6sll: add mmdc1 ipg clock

i.MX6SLL has MMDC1 ipg clock in CCM CCGR, add it 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>