xen/arm: Ignore write to GICD_ISACTIVERn registers (vgic-v2)

xen/arm: Ignore write to GICD_ISACTIVERn registers (vgic-v2)

Guests attempt to write into these registers on resume (for example Linux).
Without this patch a data abort exception will be raised to the guest.
This patch handles the write access by ignoring it, but only if the value
to be written is zero. This should be fine because reading these registers
is already handled as 'read as zero'.

Signed-off-by: Mirela Simonovic <mirela.simonovic@aggios.com>
Reviewed-by: Julien Grall <julien.grall@arm.com>