arm64: dts: imx8mn-evk: use proper names for PMIC outputs

Use the power signal names as given in the schematics of the reference
board.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Heiko Thiery <heiko.thiery@gmail.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>