ARM: dts: imx6q-h100: use usdhc2 VSELECT

The uSDHC controller directly provides a VSELECT signal that can be
muxed to the external voltage select. Mux the VSELECT directly to avoid
using a GPIO.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>