powerpc/rtas: Drop unused export symbols

Some RTAS symbols are never used by modular code, drop their exports.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Nathan Lynch <nathanl@linux.ibm.com>
Link: https://lore.kernel.org/r/20230127111231.84294-1-mpe@ellerman.id.au