powerpc/rtas: Add for_each_rtas_function() iterator

Add a convenience macro for iterating over every element of the
internal function table and convert the one site that can use it. An
additional user of the macro is anticipated in changes to follow.

Reviewed-by: "Aneesh Kumar K.V (IBM)" <aneesh.kumar@kernel.org>
Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231212-papr-sys_rtas-vs-lockdown-v6-2-e9eafd0c8c6c@linux.ibm.com