regmap: add configurable downshift for addresses

Add an additional reg_downshift to be applied to register addresses before
any register accesses. An example of a device that uses this is a VSC7514
chip, which require each register address to be downshifted by two if the
access is performed over a SPI bus.

Signed-off-by: Colin Foster <colin.foster@in-advantage.com>
Link: https://lore.kernel.org/r/20220313224524.399947-2-colin.foster@in-advantage.com


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