Platform: IOCTL for platform-specific services

Introduce new API function in HAL and a new generic platform
service to provide the means for platforms to implement proprietary
functions using a uniform framework and a single secure entry point.

Change-Id: I72ddbb3342ae851fe9851ccb27a54c8f4322b01f
Signed-off-by: Miklos Balint <miklos.balint@arm.com>