mmc: require explicit support for high-speed

mmc: require explicit support for high-speed

The new high-speed timings are similar to each other and the old
system, but not identical. And although things "just work" most of
the time, sometimes it does not. So we need to start marking which
hosts are known to fully comply with the new timings.

Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>