mmc: tmio: add flag to reduce delay after changing clock status

The docs for RCar Gen2 & 3 I have access to, mention delays of 5ms after
stop and 1ms after start. Make it possible to apply these values.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>