gpiobus(4): Add an acpi variant of gpiobus

This currently only implements the address space handler and attempts to
configure pins with flags obtained from ACPI.

Reviewed by: wulf
MFC after: 1 month
Pull Request: https://github.com/freebsd/freebsd-src/pull/1359

(cherry picked from commit 92adaa5862d5ea94318a011e0618622d0fb72521)