powerpc: Remove obsoleted declaration for _get_SP

The implementation of _get_SP() was removed in commit f4db1967
("[POWERPC] Remove  _get_SP"), remove the now obsolete declaration.

Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
[mpe: Update change log to refer to correct commit per Christophe]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240822130609.786431-2-cuigaosheng1@huawei.com