arm64: dts: imx8mm-kontron: Add support for reading SD_VSEL signal

This fixes the LDO5 regulator handling of the pca9450 driver by
taking the status of the SD_VSEL into account to determine which
configuration register is used for the voltage setting.

Even without this change there is no functional issue, as the code
for switching the voltage in sdhci.c currently switches both, the
VSELECT/SD_VSEL signal and the regulator voltage at the same time
and doesn't run into an invalid corner case.

We should still make sure, that we always use the correct register
when controlling the regulator. At least in U-Boot this fixes an
actual bug where the wrong IO voltage is used.

Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
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Changes for v2:
* rebase to current master