port92: handle A20 IRQ as GPIO

The port92 device has outgouing IRQ line A20. Currently the IRQ is referenced
by a pointer which normally is set during machine initialization. The
pointer is never changed at runtime. Hence, common GPIO model can be applied
to A20 IRQ line. Note that checking for IRQ to be connected as in
previous version of code is not required qemu_set_irq will do it.

Signed-off-by: Efimov Vasily <real@ispras.ru>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
