arm64: dts: imx8mp-kontron: Drop vmmc-supply to fix SD card on SMARC eval carrier

arm64: dts: imx8mp-kontron: Drop vmmc-supply to fix SD card on SMARC eval carrier

The SMARC evaluation carrier provides an SD card power switch that
complies with the OSM standard definition. The OSM base devicetree
already describes this correctly.

Stop overriding the vmmc-supply in the board devicetree and rely on
the definition from the OSM base DTS instead to fix the power supply
configuration for the SD card.

Fixes: 6fe1ced5ccab7 ("arm64: dts: Add support for Kontron i.MX8MP SMARC module and eval carrier")
Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: Frank Li <Frank.Li@nxp.com>