xen/arm: vgic-v2: Report the correct GICC size to the guest

xen/arm: vgic-v2: Report the correct GICC size to the guest

The GICv2 DT node is usually used by the guest to know the address/size
of the regions (GICD, GICC...) to map into their virtual memory.

While the GICv2 spec requires the size of the GICC to be 8KB, we
correctly do an 8KB stage-2 mapping but erroneously report 256 in the
device tree (based on GUEST_GICC_SIZE).

I bet we didn't see any issue so far because all the registers except
GICC_DIR lives in the first 256 bytes of the GICC region and all the
guests I have seen so far are driving the GIC with GICC_CTLR.EIOmode =
0.

Signed-off-by: Julien Grall <julien.grall@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
[ ijc -- fixed some typos in commit message ]