arm64: dts: imx8: add a node label to ddr-pmu

The ddr-pmu on i.mx8dxl has a different interrupt number.
Add a node label to ddr-pmu so that it could be referred
and changed in i.mx8dxl dts.

Signed-off-by: Shenwei Wang <shenwei.wang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>