mmc: exynos_dw_mmc: Refactor fixed CIU clock divider

mmc: exynos_dw_mmc: Refactor fixed CIU clock divider

Some chips like Exynos4412 have fixed internal CIU clock divider.
Instead of reading it from non-standard "div" dts property, store its
value in the driver internally, in static chip data associated with
corresponding compatible. This makes it possible to avoid using
host->div for storing it, so the latter can be removed safely. Also
create a helper function called exynos_dwmmc_get_ciu_div() for getting
the current div value: in case the fixed div is provided in the chip
data it will be used, otherwise the current div value is being read from
CLKSEL register.

The insights for this change were taken from dw_mmc-exynos.c driver in
Linux kernel.

No functional change.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>