refactor(notifications): verbose validity check

Make the validity check for the following interfaces
more explicit, and accompanied of useful logs:
- FFA_NOTIFICATION_SET.
- FFA_NOTIFICATION_BIND/UNBIND.

Signed-off-by: J-Alves <joao.alves@arm.com>
Change-Id: I8e8745a0deb4aa00fa1e64386d1501cd2fe52463
diff --git a/src/api.c b/src/api.c
index 95a2810..a6f03a0 100644
--- a/src/api.c
+++ b/src/api.c
@@ -938,6 +938,7 @@
 	vcpu->regs_available = true;
 	sl_unlock(&vcpu->lock);
 }
+
 /**
  * Assuming that the arguments have already been checked by the caller, injects
  * a virtual interrupt of the given ID into the given target vCPU. This doesn't