mmc: sdhci-s3c: Support controllers with no internal clock divider

This patch supports controllers with no internal clock divider in SDHCI,
such as the s5pc210 board.  The external clock divider can be used to
make a proper clock because SDHCI doesn't support internal clock divider
by itself.

If external clock divider type is selected, some functions related
to clock control will be overrided by other functions.

The current clock control index is added to let you know which
clock bus is used for SDHCI when overriding functions.

Checking functions are added into sdhci_s3c_consider_clock, because
the clock divider step is different from that of host controller.

Signed-off-by: Jeongbae Seo <jeongbae.seo@samsung.com>
Cc: Jaehoon Chung <jh80.chung@samsung.com>
Cc: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Chris Ball <cjb@laptop.org>