axp81x: Do not fail if regulators aren't properly defined

If a regulator is missing a mandatory property (like 'regulator-name'), do
not fail, regulator_parse_ofw_stdparam is returning a non-zero value so just
skip this regulator.
Also if any regulator fails to attach continue with the rest of the regulators
instead of returning ENXIO in axp8xx_attach

Tested On: BananaPi M3