Use kunit_device_register() to create a real struct device for the
regmap instead of leaving it at NULL.

The main reason for this is that it allows context data to be passed
into the readable_reg/writable_reg/volatile_reg functions by attaching
it to the struct device with dev_set_drvdata().

The gen_regmap() and gen_raw_regmap() functions are updated to take a
struct kunit * argument.

A new struct regmap_test_priv has been created to hold the struct device
created by kunit_device_register(). This allows the struct to be
extended in the future to hold more private data for the test suite.
Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Link: https://msgid.link/r/20240408144600.230848-3-rf@opensource.cirrus.comSigned-off-by: Mark Brown <broonie@kernel.org>