regulator: fixed: support deferred probe for DT GPIOs

of_get_named_gpio() needs the driver hosting the GPIO that the DT
property references to have been probed. Detect this specific failure,
and defer the probe of the whole regulator until this API can complete.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>