arm64: dts: imx8mm: update pmu compatible

i.MX8MM features four Cortex-A53 cores, update the compatible
to use more accurate "arm,cortex-a53-pmu"

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>