xen: arm: implement cpuinfo

xen: arm: implement cpuinfo

Use to:

 - Only context switch ThumbEE state if the processor implements it. In
   particular the ARMv8 FastModels do not.
 - Detect the generic timer, and therefore call identify_cpu before
   init_xen_time.

Also improve the boot time messages a bit.

I haven't added decoding for all of the CPUID words, it seems like overkill
for the moment.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Tim Deegan <tim@xen.org>
Cc: stefano.stabellini@citrix.com