SPM: Update service structure

Prepare for partition storage arrangement, normalize the
structure name:

- Change the service structure name to 'service_t'
- Change the global service lists name to 'g_services'

Change-Id: Ic35c9b27f03054c46127b3453e71d70da78e05f6
Signed-off-by: Mingyang Sun <mingyang.sun@arm.com>
5 files changed