mmc: mmci: add set_clk/pwrreg callbacks

This patch adds set_clkreg and set_pwrreg callbacks
at mmci_host_ops to allow to call specific variant.
extends visibility of mmci_write_clk/pwrreg functions
to be used into specific file variant.

Signed-off-by: Ludovic Barre <ludovic.barre@st.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>