A pointer to rt_svc_desc_t is defined both in the function
handle_runtime_svc() and globally. Since the value of the
pointer RT_SVC_DESCS_START is defined by the linker and
never changes make this definition local in both
handle_runtime_svc() and runtime_svc_init() to reduce the
number of loads

Change-Id: Iea42c778d8599a26c87700009163b5a8d7d60be2
Signed-off-by: Daniel Boulby <daniel.boulby@arm.com>