regmap: Provide test for regcache_reg_present()

Provide a KUnit test for the newly added API.

Reviewed-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230717-regmap-cache-check-v1-2-73ef688afae3@kernel.org


Signed-off-by: Mark Brown <broonie@kernel.org>