regmap: Add support for 10/14 register formating

This patch adds support for 10 bits register, 14 bits value type register
formating. This is for example used by the Analog Devices AD5380.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>