ARM: dts: stm32: Enable voltage translator auto-detection on DHCOM

The DHCOM SoM uSD slot has an optional voltage level translator, add
DT bindings which permit the MMCI driver to detect the translator
automatically.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Ludovic Barre <ludovic.barre@st.com>
Cc: Ulf Hansson <ulf.hansson@linaro.org>
Cc: linux-stm32@st-md-mailman.stormreply.com
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>