Fix handler prototype in Trusty generic dispatcher

Fix the types of the arguments of trusty_generic_platform_smc()
to match the expected prototype of a runtime service handler
(see rt_svc_handle_t type).

Change-Id: Ie839d116ca924b4b018ea2abbef72a1073da2a32
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>