qoriq_gpio: Add support for driver inheritance

Expose softc and other necessary things in a separate header.
This is needed for an armv8 specific driver, that will inherit from this
one. Driver mutex was converted to a spin lock, so that it can be later
used in interrupt filter context.

Obtained from: Semihalf
Sponsored by: Alstom Group
Differential revision: https://reviews.freebsd.org/D32587