power_supply: Add battery health reporting for WM8350

Implement support for reporting battery health in the WM8350 battery
interface. Since we are now able to report this via the classs remove
the diagnostics from the interrupt handler.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Anton Vorontsov <cbouatmailru@gmail.com>
Signed-off-by: Samuel Ortiz <sameo@openedhand.com>