ARM: dts: stm32: rename mmc_vcard to vcc-3v3 on stm32f746-disco

ARM: dts: stm32: rename mmc_vcard to vcc-3v3 on stm32f746-disco

In the schematics of document UM1907, the power supply for the micro SD
card is the same 3v3 voltage that is used to power other devices on the
board. By generalizing the name of the voltage regulator, it can be
referenced by other nodes in the device tree without creating
misunderstandings.

This patch is preparatory for future developments.

Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>