docs/s390x/pcidevices: document pci devices on s390x

docs/s390x/pcidevices: document pci devices on s390x

Add some documentation about the zpci device and how
to use it with pci devices on s390x.

Used source: Cornelia Huck's blog post
https://people.redhat.com/~cohuck/2018/02/19/notes-on-pci-on-s390x.html

Signed-off-by: Sebastian Mitterle <smitterl@redhat.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Message-Id: <20230127123349.55294-1-smitterl@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>