feat(ffa-svc): add `ffa_func_name` and `ffa_error_name`

Adds helper functions for getting string representations of function
identifiers and error codes. This makes debug logs more readable than
printing the integer value.

Change-Id: I9e74c197686dc08e0c71886f641c60829587bad6
Signed-off-by: Karl Meakin <karl.meakin@arm.com>
7 files changed