regmap: Add KUnit tests for read/write checking

Merge series from Mark Brown <broonie@kernel.org>:

Since Takashi found an issue with maple tree syncing registers it
shouldn't do add some test cases that catch that case and some more
potential issues, ideally we'd run through the combination of
readability with all possible I/O calls but that's lifting for another
day.  We did find one issue with missing readability checks which will
be fixed separately.