MIPS: mark prom_free_prom_memory() everywhere with __init

On OCTEON the function is non-trivial and we can potentially even save
some memory.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Acked-by: David Daney <david.daney@cavium.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/9338/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>