regmap: Provide basic test coverage for raw I/O

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

Our existing coverage only deals with buses that provide single register
read and write operations, extend it to cover raw buses using a similar
approach with a RAM backed register map that the tests can inspect to
check operations.  This coverage could be more complete but provides a
good start.

diff --cc drivers/base/regmap/internal.h
Simple merge