util: Add functions for s390x mmio read/write

util: Add functions for s390x mmio read/write

Starting with z15 (or newer) we can execute mmio
instructions from userspace. On older platforms
where we don't have these instructions available
we can fallback to using system calls to access
the PCI mapped resources.

This patch adds helper functions for mmio reads
and writes for s390x.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Farhan Ali <alifm@linux.ibm.com>
Acked-by: Thomas Huth <thuth@redhat.com>
Message-id: 20250430185012.2303-2-alifm@linux.ibm.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>