Merge the QorIQ GPIO drivers between arm and powerpc

Summary:
They're nearly identical, so don't use two copies. Merge the newer
driver into the older one, and move it to a common location.

Add the Semihalf and associated copyrights in addition to mine, since
it's a non-trivial amount of code merged.

Reviewed By: mw
Differential Revision: https://reviews.freebsd.org/D29520