regmap: add helper for per-port regfield initialization

Similar to the standalone regfields, add an initializer for the users
who need to set .id_size and .id_offset in order to use the
regmap_fields_update_bits_base API.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20200527234113.2491988-2-olteanv@gmail.com


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