mmc: dw_mmc: exynos: add the function for controlling SMU

Some of Exynos has the Security management Unit(SMU).
This patch adds the function for controlling SMU.

In future, if exynos needs to control SMU, it can be implemented
in "config_smu" function, not "init" function.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>