The U-Boot SDIO driver expects the SD interface to operate at reference clock of 50MHz.
If the reference clock is lower than 50MHz, the clock divisor setting will not be configured correctly.
This can be worked around with the attached path.
The patch can be applied to PetaLinux 2014.4 and 2015.1.
Apply the patch to a cloned copy of the U-Boot source code using the following:
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