clk: imx: keep the mmdc p1 ipg clock always on on 6sx/ul/ull/sll

The MMDC_P1_IPG clock need to be on always on to make sure
the MMDC register can be accessed successfully.

Signed-off-by: Jacky Bai <ping.bai@nxp.com>
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>