An SPI driver for Zynq platforms

[skibo]

Summary

This is an SPI driver for the Xilinx Zynq chip.

Test Plan

I have tested this driver on a Zybo board using an SPI-to-ethernet device attached to a PMOD interface. I did not write a real driver for this ethernet device, I simply read and wrote registers and made modest data transfers back and forth to the device using the spigen driver.