spi: add loopback test driver to allow for spi_master regression tests

This driver is submitting lots of distinct spi-messages messages
with all kinds of alignments and length pattern.
Also distinct kinds of transfer pattern tests are implemented
(rx, tx, rx/tx, tx+tx, tx+rx,...)

Right now on a raspberry pi 752 distinct spi_messages are executed
in 13 different scenarios.

Configuration of additional test-pattern is easy, so that when
new bugs in drivers get detected the relevant transfer pattern can
also get added to the test framework, so that such situations are
detected in other drivers as well.

The idea behind this driver is to make it possible to also detect
regressions in spi_master implementations when changes occur.
Potentially these tests could get executed automatically in a
test-server-farm.

Signed-off-by: Martin Sperl <kernel@martin.sperl.org>
Signed-off-by: Mark Brown <broonie@kernel.org>