MIPS: Octeon: remove orphan octeon_swiotlb declaration

All uses of octeon_swiotlb were removed by
commit f39f8d0e ("MIPS/octeon: use swiotlb_init instead
of open coding it"), so remove the orphan declaration, too.

Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>