xen/arm: Save/Restore GICH_APR register

xen/arm: Save/Restore GICH_APR register

Linux uses GICC_CTLR.EOImodeNS set to 0, which means both priority drop and
deactivate interrupt functionality are made when something is written in
GICC_EOIR.

As the ARM manual specifies: "having an active interrupt in the List registers
with a priority that is not set in the corresponding Active Priorities
register" when GICV_CTLR.EOImode (ie GICC_CTLR.EOImodeNS in the guest context)
result in unpredicable behavior, we need to save/restore GICH_APR.

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