xen/arm: vgic-v2: Implement correctly ITARGETSR0 - ITARGETSR7 read-only

xen/arm: vgic-v2: Implement correctly ITARGETSR0 - ITARGETSR7 read-only

Each ITARGETSR register are 4-byte wide and the offset is in byte.

The current implementation is computing the end of the range wrongly
resulting to emulate only ITARGETSR{0,1} read-only. The rest will be
treated as read-write.

As 8 registers should be read-only, the end of the range should be
ITARGETSR + (4 * 8) - 1.

For convenience introduce ITARGETSR7 and ITARGETSR8.

Signed-off-by: Julien Grall <julien.grall@citrix.com>
Reviewed-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>