mmc: stm32_sdmmc2: Add support for probing bus voltage level translator

Add support for testing whether bus voltage level translator is present
and operational. This is useful on systems where the bus voltage level
translator is optional, as the translator can be auto-detected by the
driver and the feedback clock functionality can be disabled if it is
not present.

The translator test sets CMD high to avoid interfering with a card, and
then verifies whether signal set on CK is detected on CKIN. If the signal
is detected, translator is present, otherwise the CKIN feedback clock are
disabled.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Patrice Chotard <patrice.chotard@foss.st.com>
Cc: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Yann Gautier <yann.gautier@foss.st.com>