x86: Templatize IntSlavePort.

x86: Templatize IntSlavePort.

This makes the device IntSlavePort calls back into based on a template
parameter so that IntDevice doesn't have to be in the inheritance
hierarchy to use it.

It also makes IntSlavePort inherit from SimpleTimingPort directly,
skipping over MessageSlavePort.

Change-Id: Ic3213edc9c3ed5e506ee1e9f5e082cd47d7c7998
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20820
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>