ASoC: ak5386: Add regulators to documentation and fix sparse warning

Document the newly added regulators to the DT binding document.

Also, "static const char const *x" is not identical to "static const
char * const x", which sparse now complains about. Fix it.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Mark Brown <broonie@linaro.org>