regmap: add missing dependency on SoundWire

CONFIG_REGMAP is not selected when no other serial bus is supported.
It's largely academic since CONFIG_I2C is usually selected e.g. by
DRM, but still this can break randconfig so let's be explicit.

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20200707202628.113142-1-pierre-louis.bossart@linux.intel.com


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