I see that there are USB wall outlets now available. These are meant for use in charging electronic devices with only a USB cable (no little "brick" is needed). I'm wondering whether these are OK to use to charge iPads? I have a mini, an iPad4 and an iPad Pro. In particular, the IPP's "brick" seems to be slightly larger than the others, so I'm not sure whether it requires something special or proprietary, or whether they all can be safely charged from a "generic" USB wall port? Any thoughts? Thanks!