regmap: add a helper to translate the register address

Register addresses passed to regmap operations can be offset with
regmap.reg_base and downshifted with regmap.reg_downshift.

Add a helper to apply both these operations and return the translated
address, that we can then use to perform the actual register operation
ont the underlying bus.

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Link: https://lore.kernel.org/r/20230324093644.464704-2-maxime.chevallier@bootlin.com


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