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.

Change-Id: I4bcc9313a4f4524b21124a4c1024bdbe26221fc3
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>