regmap: add reg_sequence helpers

Add helper to make it easier to define a reg_sequence array.

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Link: https://lore.kernel.org/r/20200402084111.30123-1-m.felsch@pengutronix.de


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