arm64: dts: imx8mm-phycore: drop uncorrect vselect-en

There is no "vselect-en" property, neither in the bindings nor in the
Linux driver:

  imx8mm-phyboard-polis-rdk.dtb: pmic@8: regulators:ldo2: Unevaluated properties are not allowed ('vselect-en' was unexpected)

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Yannic Moog <y.moog@phytec.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>