tmio_mmc: Remove const from platform data V3

Remove const from the tmio-mmc platform data hclk V3.
This change makes it possible to remove the type cast
from the sh_mobile_sdhi driver which is using the clock
framework to get the clock rate.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>