regmap: Add stub regmap calls as a build crutch for infrastructure users

Make life easier for subsystems which build infrastructure on top of the
regmap API by providing stub definitions for most of the API so that users
can at least build even if the regmap API is not enabled without having to
add ifdefs if they don't want to.

These stubs are not expected to be useful and should never actually get
called, they just exist so that we can link so there are WARN_ONCE()s in
the stubs.

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