mmc: zynq: Add fdt max-frequency support

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 <[email protected]>
Acked-by: Michal Simek <[email protected]>
Reviewed-by: Jaehoon Chung <[email protected]>
Signed-off-by: Michal Simek <[email protected]>