The FF-A driver operations are currently not accessible from user
space. This commit adds a thin layer using a DebugFS interface to
expose them. It is a temporary workaround for the OP-TEE team,
prototyping S-EL0 Secure Partitions and OP-TEE as S-EL1 SPMC.

Change-Id: Ic15a3acf07d74139b37af4aee3a69de634056ba6
Signed-off-by: Balint Dobszay <balint.dobszay@arm.com>