ARM: dts: imx6qdl-kontron-samx6i: add SDIO_PWR_EN support

The module can enable/disable the SD card power supply and has
it's dedicated pin SDIO_PWR_EN for that reason. This is esp.
useful to be able to reset the SD card if it was in an UHS mode.

Signed-off-by: Michael Walle <mwalle@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>