docs: Ensure Doxygen return values use references

At present a lot of the code-base uses `\retval` commands to framework
status codes but does not explicitly link to them. In some cases it's
necessary to explicitly link to these status codes to have Doxygen
generate a link to the relevant enumeration, and we can do this by
prefixing the name with `::`.

This style is already used in much of the code-base, so should not
conflict with any existing styles.

Change-Id: Ifc78eb425981bee3a01c791757a41a61c20e636c
Signed-off-by: Chris Kay <chris.kay@arm.com>