Adding support on the microblaze software for XpsLoopback test - PC sends 4KB block of data, microblaze DMAs from input to output buffer in spiifc, PC reads data back

Although 27.9mbps is possible at 100 MHz clock, switching down to 22mbps for microblaze 66.67 MHz clock

Adding more SPI master test scripts, including loopback, which verifies the non-XPS implementation works at up to 28mbps

Adding more SPI master test scripts, including loopback, which verifies the non-XPS implementation works at up to 28mbps

Signed-off-by: Mike Lyons <github@explosivedonut.com>

Refactoring spitest to separate SPI commands from the test. Will create a separate module for SPI commands next.