SPI: zynq: Created separate API for clock change

Moved clock polarity, phase and frequency changes to a separate
API 'zynq_spi_config_clock' from zynq_spi_setup_transfer.

Signed-off-by: Harini Katakam <harinik@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>