spi: zynq_spi: Use clk subsystem to get reference spi clk

spi: zynq_spi: Use clk subsystem to get reference spi clk

Remove fixed reference clk used by plat->frequency and use clk
subsystem to get reference clk. As per spi dt bindings
"spi-max-frequency" property should be used by the slave devices.
This property is read by spi-uclass driver for the slave device.
So avoid reading above property from the platform driver.

Signed-off-by: T Karthik Reddy <t.karthik.reddy@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>