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.