mmc: mmci: Share sdmmc_variant_init() via the common header file

It's good practice to share functions via header files, rather than from
the c-files. Therefore, let's move sdmmc_variant_init() to mmci.h.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Ludovic Barre <ludovic.barre@st.com>
Tested-by: Ludovic Barre <ludovic.barre@st.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>