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>

arch/mips/mti-sead3/sead3-init.c

@@ -147,6 +147,6 @@ void __init prom_init(void)
 #endif
 }
 
-void prom_free_prom_memory(void)
+void __init prom_free_prom_memory(void)
 {
 }