arm64: dts: imx8mn: Add LDO5 regulator-name to Variscite VAR-SOM-MX8MN

The PMIC on this Variscite SOM and its MX8MM variant lists regulator-name
for all LDOs except this LDO5, add the regulator-name property to avoid
this omission.

Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>