[PATCH] mmc: Add support for mmc v4 high speed mode

This adds support for the high-speed modes defined by mmc v4
(assuming the host controller is up to it). On a TI sdhci controller,
it improves read speed from 1.3MBps to 2.3MBps. The TI controller can
only go up to 24MHz, but everything helps. Another person has taken
this basic patch and used it on a Nokia 770 to get a bigger boost
because that controller can run at 48MHZ.

Signed-off-by: Philip Langdale <philipl@overt.org>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>