ARM: OMAP: hsmmc: add max_freq field

External circuitry like level shifters may limit the maximum operation
speed of the hsmmc controller. Add a field to struct omap2_hsmmc_info
so boards can adjust the setting on demand.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Chris Ball <cjb@laptop.org>