clk: imx8mm: Mark dram_apb critical

This clock is used for dram operations inside TF-A and must be kept
enabled for features such as suspend/resume dram retention and busfreq
to work.

This is required for imx8mm suspend to work with NXP branch of TF-A.
There is an equivalent clk on imx8mq and it's always been marked as
critical in upstream.

Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>