regmap: Provide device read and write map interface for merging

Add the externally visible interface introduced by Lars-Peter's commit
6f3064 (regmap: Add support for device specific write and read flag
masks) separately in order to allow merge into other subsystems for
integration with drivers.  Drivers relying on this feature will not be
functional until they are merged with the implementation.

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