xen/arm32: Introduce lookup_processor_type

xen/arm32: Introduce lookup_processor_type

Looking for a specific proc_info structure is already implemented in assembly.
Implement lookup_processor_type to avoid duplicate code between C and
assembly.

This function searches the proc_info_list structure following the processor
ID. If the search fail, it will return NULL, otherwise a pointer to this
structure for the specific processor.

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