mmc: tmio/sdhi: distinguish between SCLKDIVEN and ILL_FUNC

This bit has a different meaning in SDHI and original TMIO. Document
that and use the proper naming.

Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>