mmc: omap_hsmmc: add DT property for max bus frequency

Maximum bus frequency can be limited by external circuitry like level
shifters etc. Allow passing this value from DT.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Venkatraman S <svenkatr@ti.com>
Signed-off-by: Chris Ball <cjb@laptop.org>