mmc: Added Kconfig support for CONFIG_ZYNQ_SDHCI_MIN_FREQ

This patch added Kconfig support for CONFIG_ZYNQ_SDHCI_MIN_FREQ
and enabled it in respective defconfig.

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