Fix semaphore clean up in RProc destructor

The semaphore created in the RProc constructor is never cleaned up. To
resolve this, the destructor will now delete the semaphore.

Change-Id: Ib641a87129bacf3d7ba097c4e89aa77fa255e2f8
Signed-off-by: Mikael Olsson <mikael.olsson@arm.com>