ARM: dts: imx6q-bx50v3: disable SD card

Default boot medium must be internal eMMC, we don't want the SDIO to be
used for these devices.  This commit disables the SD card interface for
GE Healthcare Bx50v3 devices.

Signed-off-by: Ian Ray <ian.ray@ge.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>