regulator: core: Add new API to poll for error conditions

Regulator consumers can receive event notifications when
errors are reported to the driver, but currently, there is
no way for a regulator consumer to know when the error is over.

To allow a regulator consumer to poll for error conditions
add a new API: regulator_get_error_flags.

Signed-off-by: Axel Haslam <ahaslam@baylibre.com>
Signed-off-by: Mark Brown <broonie@kernel.org>