amdgpio: Mask and service interrupts

Mask all interrupts coming from the AMD GPIO controller and service any
potential interrupts. Unserviced interrupts can block entry to S0i3 on
certain AMD CPUs.

Reviewed by: aokblast, mckusick (mentor)
Approved by: aokblast, mckusick (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D51588