xen: arm: allow platform code to select dom0 event channel irq

xen: arm: allow platform code to select dom0 event channel irq

Currently the hardcoded use of GUEST_EVTCHN_PPI is problematic if that is a
real PPI on the platform.

We really need to be smarter about selecting an unused PPI but in the meantime
we can at least give the platform code the option of hardcoding a number which
works for the platform.

Hardcode a suitable PPI on the Xgene platform.

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>