regmap: REGMAP_KUNIT should not select REGMAP

Enabling a (modular) test should not silently enable additional kernel
functionality, as that may increase the attack vector of a product.

Fix this by:
  1. making REGMAP visible if CONFIG_KUNIT_ALL_TESTS is enabled,
  2. making REGMAP_KUNIT depend on REGMAP instead of selecting it.

After this, one can safely enable CONFIG_KUNIT_ALL_TESTS=m to build
modules for all appropriate tests for ones system, without pulling in
extra unwanted functionality, while still allowing a tester to manually
enable REGMAP and its test suite on a system where REGMAP is not enabled
by default.

Fixes: 2238959b

 ("regmap: Add some basic kunit tests")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/b0a5dbb17c1d5ea482e052e585ae83bb69c48806.1682516005.git.geert@linux-m68k.org
Signed-off-by: Mark Brown <broonie@kernel.org>