arm64: dts: imx8mp-phycore-som: Remove invalid PMIC property

'regulator-compatible' is not a valid property according to
nxp,pca9450-regulator.yaml and causes the following warning:

  DTC_CHK arch/arm64/boot/dts/freescale/imx8mp-dhcom-pdk2.dtb
...
pmic@25: regulators:LDO1: Unevaluated properties are not allowed ('regulator-compatible' was unexpected)

Remove the invalid 'regulator-compatible' property.

Cc: Teresa Remmet <t.remmet@phytec.de>
Fixes: 88f7f6bc ("arm64: dts: freescale: Add support for phyBOARD-Pollux-i.MX8MP")
Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Teresa Remmet <t.remmet@phytec.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>