regulator: Fixes for v3.15

A couple of things here:

 - Fixes for pbias that didn't make it in during the merge window due to
   the driver coming in via MMC.  The conversion to use helpers is a
   fix as it implements list_voltage() which the main user (MMC) relies
   on for correct functioning.
 - Change the !REGULATOR stub for optional regulators to return an
   error rather than a dummy; this is more in keeping with the intended
   use of optional regulators and fixes some issues seen MMC where it
   got confused by a dummy being provided.
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