When the bus switches to 1.8V mode of operation, it is necessary to
verify that the card correctly initiated and completed the voltage
switch. This is done by reading out the state of DATA0 line.

This patch implement support for reading out the state of the DATA0
line, so the MMC core code can correctly switch to 1.8V mode.
Signed-off-by: NMarek Vasut <marek.vasut+renesas@gmail.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>