mmc: zynq: Add fdt max-frequency support

The maximum supported peripheral clock frequency of the zynq depends on
the IO routing. The MIO and EMIO support a maximum frequency of 50 MHz
respectively 25 MHz. Use the max-frequency value of the device tree to
determine the maximal supported peripheral clock frequency.

Signed-off-by: Stefan Herbrechtsmeier <stefan.herbrechtsmeier@weidmueller.com>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>