mmc: Define timing macro's

Define timing macro's for all the available speeds of mmc. This is
done similar to linux. Replace speed macro's used with these new timing
macro's wherever applicable.

Signed-off-by: Ashok Reddy Soma <ashok.reddy.soma@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>