arm64: dts: imx8mm-evk: Align regulator names with schema

Device tree schema expects regulator names to be lowercase.  This fixes
dtbs_check warnings like:

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

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>