dm: spi: zynq_spi: Convert to driver model

This converts the zynq spi driver to use the driver model.

Minimal functional changes like using meaningful name on
structure members wrt mainlined dm spi drivers.
- input_hz -> frequency
- req_hz -> freq
- base -> regs

Signed-off-by: Jagan Teki <jteki@openedev.com>
Acked-by: Simon Glass <sjg@chromium.org>
Cc: Michal Simek <michal.simek@xilinx.com>
Cc: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Tested-by: Jagan Teki <jteki@openedev.com>