If variant hasn't the control bit to switch pads in opendrain mode,
we can achieve the same result by asking to the pinmux driver to
configure pins for us.

This patch make the mmci driver able to do this whenever needed.
Signed-off-by: NAndrea Merello <andrea.merello@gmail.com>
Signed-off-by: NPatrice Chotard <patrice.chotard@st.com>
Signed-off-by: NUlf Hansson <ulf.hansson@linaro.org>