xen/arm: vgic-v3: Don't try to emulate IROUTER which do not exist in the spec

xen/arm: vgic-v3: Don't try to emulate IROUTER which do not exist in the spec

The range of valid IROUTER<n> are n = 32 - 1019 (see 8.9.13 in IHI 0069A)
which correspond to the offset 0x6100-0x7FD8.

Other offsets are invalid and therefore should not be emulated.

Also remove the now unused label read_as_zero_64 and write_ignore_64.

Note that GICD_IROUTER is kept to accommodate the GICv3 drivers which has
been in part taken from Linux.

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