Add a GPIO driver for the Arm pl061 controller

A PL061 is a simple 8 pin GPIO controller. This GPIO device is used to
signal an internal request for shutdown on some virtual machines including
Arm-based Amazon EC2 instances.

Submitted by: Ali Saidi <alisaidi_amazon.com> (previouss version)
Reviewed by: Ali Saidi, manu
Differential Revision: https://reviews.freebsd.org/D24065