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zynq: sdhci: Calculate minimum frequency based on max frequency

Calculate the minimum sd clock based on max clock. This will
be done by add_sdhci() if we pass minimum clock as zero.
It also does based on SD host contoller version.

Signed-off-by: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>