mmc: mmci: Add private pointer for variant

In variant init function, some references may be allocated for variant
specific usage. Add a private void* to mmci_host struct allows at variant
functions to access on this references by mmci_host structure.

Signed-off-by: Ludovic Barre <ludovic.barre@st.com>
Link: https://lore.kernel.org/r/20200128090636.13689-5-ludovic.barre@st.com


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>