regmap: Allow devices to specify which registers are accessible

This is currently unused but we need to know which registers exist and
their properties in order to implement diagnostics like register map
dumps and the cache features.

We use callbacks partly because properties can vary at runtime (eg, through
access locks on registers) and partly because big switch statements are a
good compromise between readable code and small data size for providing
information on big register maps.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>