regulator: Support driver probe deferral

If we fail to locate a requested regulator return -EPROBE_DEFER. If drivers
pass this error code through to their caller (which they really should)
then this will ensure that the probe is retried later when further devices
become available.  In the unusual case where a driver doesn't want this
it can override the default behaviour.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>