mmc: sdhci-s3c: Provide documentation for missing struct properties

mmc: sdhci-s3c: Provide documentation for missing struct properties

Describe properties; ext_cd_irq, clk_rates and no_divider (x2).

Squashes the following W=1 kernel build warnings:

 drivers/mmc/host/sdhci-s3c.c:126: warning: Function parameter or member 'ext_cd_irq' not described in 'sdhci_s3c'
 drivers/mmc/host/sdhci-s3c.c:126: warning: Function parameter or member 'clk_rates' not described in 'sdhci_s3c'
 drivers/mmc/host/sdhci-s3c.c:126: warning: Function parameter or member 'no_divider' not described in 'sdhci_s3c'
 drivers/mmc/host/sdhci-s3c.c:139: warning: Function parameter or member 'no_divider' not described in 'sdhci_s3c_drv_data'

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ben Dooks <ben-linux@fluff.org>
Cc: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Link: https://lore.kernel.org/r/20200701124702.908713-5-lee.jones@linaro.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>