arm64: dts: imx8mn-ddr4-evk: Align regulator names with schema

Device tree schema expects regulator names to be lowercase.  Changing to
lowercase has multiple effects:
1. LDO6 supply is now properly configured, because regulator driver
   looks for supplies by lowercase name,
2. User-visible names via sysfs or debugfs are now lowercase,
2. dtbs_check warnings are fixed:

    pmic@4b: regulators:LDO1:regulator-name:0: 'LDO1' does not match '^ldo[1-6]$'

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-By: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>