ARM64: zynqmp: ep: Define minimum sdhci frequency for ep

Define minimum sdhci frequency for ep, as not defining
it causes the divisor to be 2048 as per sd version but
keeping clock very low on ep causes command failures.

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