powerpc/rtas: make all exports GPL

powerpc/rtas: make all exports GPL

The first symbol exports of RTAS functions and data came with the (now
removed) scanlog driver in 2003:

https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git/commit/?id=f92e361842d5251e50562b09664082dcbd0548bb

At the time this was applied, EXPORT_SYMBOL_GPL() was very new, and
the exports of rtas_call() etc have remained non-GPL. As new APIs have
been added to the RTAS subsystem, their symbol exports have followed
the convention set by existing code.

However, the historical evidence is that RTAS function exports have been
added over time only to satisfy the needs of in-kernel users, and these
clients must have fairly intimate knowledge of how the APIs work to use
them safely. No out of tree users are known, and future ones seem
unlikely.

Arguably the default for RTAS symbols should have become
EXPORT_SYMBOL_GPL once it was available. Let's make it so now, and
exceptions can be evaluated as needed.

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Reviewed-by: Laurent Dufour <laurent.dufour@fr.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20230124140448.45938-3-nathanl@linux.ibm.com