ASoC: Add macros for defining virtual enums

With the upcoming consolidation of normal MUXs and virtual MUXs we need to be
able to distinguish between enums with and without a backing register at the
enum level. Use the same approach as used for virtual mixer controls by setting
the reg field of the enum to SND_SOC_NOPM for enums without a backing register.
This patch adds a set of helper macros that can be used to define such enums.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@linaro.org>