The backup battery on WM831x is a separate IP block to the main PMU
and is largely unrelated to the main supply functionality. Factor it
out into a separate driver in order to reflect this and better support
future hardware versions.
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com> Acked-by: Samuel Ortiz <sameo@linux.intel.com> Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>