mmc: tmio: Preinitialize regulator to 3.3V

Preinitialize the SD card signals regulator to 3.3V, which is the
default post-reset setting, to be sure the regulator is set to a
valid value.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>