firmware: cirrus: Add KUnit tests for cs_dsp

Merge series from Richard Fitzgerald <rf@opensource.cirrus.com>:

This series adds KUnit tests for the cs_dsp module.

Most of the functionality in cs_dsp is for downloading firmware to
DSP memory and interacting with "control" words defined in that
memory. This doesn't require any emulation of running firmware,
because it is only reading and writing registers. So the testing can
be done using a dummy regmap. The way this is used to perform testing
is described in more detail in the commit message for each test.

ADSP1 is not tested because this was only used by the WM2200 codec,
a long-obsolete part that was discontinued in 2015.