xen/arm: ignore writes to GICD_ICACTIVER ... GICD_ICACTIVERN

xen/arm: ignore writes to GICD_ICACTIVER ... GICD_ICACTIVERN

Injecting a fault to the guest just because it is writing to one of the
GICD_ICACTIVER registers, which are part of the GICv2 and GICv3 specs,
is harsh. Additionally it causes recent linux kernels to fail to boot on
Xen.

Ignore writes to GICD_ICACTIVER ... GICD_ICACTIVERN instead, to solve
the boot issue and for backportability. However implementing the
registers properly might a better long term solution.

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