xen/arm: gic: Use the correct CPU ID

xen/arm: gic: Use the correct CPU ID

The GIC mapping of CPU interfaces does not necessarily match the logical
CPU numbering.

When Xen wants to send an SGI to specific CPU, it needs to use the GIC CPU
ID.  It can be retrieved from ITARGETSR0, in fact when this field is read,
the GIC will return a value that corresponds only to the processor reading
the register.  So Xen can use the PPI 0 to initialize the mapping.

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