xen: arm: take FIQ exceptions to Xen not guest by setting HCR_EL2.FMO

xen: arm: take FIQ exceptions to Xen not guest by setting HCR_EL2.FMO

As with HCR_EL2.{IMO,AMO} we want to route FIQs to Xen not the guest. See ARM
ARM DDI 0406C.b B1.8.4.

So far none of the platforms which we support use FIQ for anything, but when we
end up supporting one it would be far better to surprise Xen with them than
whatever guest happens to be running...

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Julien Grall <julien.grall@linaro.org>