regmap: Add support for 12/20 register formatting

regmap: Add support for 12/20 register formatting

Devices such as the AD5628 require 32 bits of data divided in 12 bits
for dummy, command and address, and 20 for data and dummy. Eg:

XXXXCCCCAAAADDDDDDDDDDDDDDDDXXXX

Where X is dont care, C is command, A is address and D is data bits.

Which would requierd the following regmap_config:

static const struct regmap_config config_dac = {
	.reg_bits = 12,
	.val_bits = 20,
	.max_register = 0xff,
};

Signed-off-by: Ricardo Ribalda <ribalda@kernel.org>
Link: https://lore.kernel.org/r/20200917114727.1120373-1-ribalda@kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>