imx: kontron-sl-mx8mm: Use the VSELECT signal to switch SD card IO voltage

imx: kontron-sl-mx8mm: Use the VSELECT signal to switch SD card IO voltage

It turns out that it is not necessary to declare the VSELECT signal as
GPIO and let the PMIC driver set it to a fixed high level. This switches
the voltage between 3.3V and 1.8V by setting the PMIC register for LDO5
accordingly.

Instead we can do it like other boards already do and simply mux the
VSELECT signal of the USDHC interface to the pin. This makes sure that
the correct voltage is selected by setting the PMIC's SD_VSEL input
to high or low accordingly.

Reported-by: Heiko Thiery <heiko.thiery@gmail.com>
Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Reviewed-by: Heiko Thiery <heiko.thiery@gmail.com>
Reviewed-by: Fabio Estevam <festevam@denx.de>