mmc: core: Add a new quirk for limiting clock rate

This patch adds a quirk to limit clock rate which
can be used to reduce the SDIO clock rate for some
chips with broken UHS.

Signed-off-by: Harish Jenny K N <harish_kandiga@mentor.com>
Reviewed-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>