arm64: dts: imx8mm: Drop deprecated regulator-compatible from Variscite VAR-SOM-MX8MM

The "regulator-compatible" property is deprecated and unused, as the
match happens on the node name in Linux of_regulator_match() in case
the property is not present. Drop the deprecated property from DT.

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