mmc: tmio/sdhi: introduce flag for RCar 2+ specific features

RCar Gen2 and later implementations of TMIO/SDHI have their own set of
features and additions. FAST_CLK_CHG is just one of them and I see a few
others being added soon. Some may work on older chipsets but this needs
to be tested case by case. Instead of adding a bunch of flags for each
feature, add a global RCar2+ one for now. We can still break out
features if the need arises.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>