arm64: dts: imx8m: Add ddr-pmu nodes

The same ddr perfomance counter IP from 8qxp is also available on imx8m
series so add it to dts.

Tested with `perf stat` and `memtester` on imx8mm-evk and obtained
plausible results.

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