feat: allow FFA_RUN as return from Waiting state

FF-A v1.1 EAC0 Figure 7.1 depicts an endpoint in Waiting state can
enter the runtime model for FFA_RUN through the ERET conduit.
This change adds this case to the cactus main loop.

Signed-off-by: Olivier Deprez <olivier.deprez@arm.com>
Change-Id: I655ac0a7240bcfd25932e1a4cf51c331baceaf47