linux-imx

nds32: Add document for NDS32 PMU.

The document for how to add NDS32 PMU
in devicetree.

Signed-off-by: Nickhu <nickhu@andestech.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Greentime Hu <greentime@andestech.com>
Signed-off-by: Greentime Hu <greentime@andestech.com>