prom_free_prom_memory() returns unsigned long

    Some boards declare prom_free_prom_memory as a void function but the
    caller free_initmem() expects a return value.

    Fix those up and return 0 instead, just like everyone else does.

    Signed-off-by: Arthur Othieno <a.othieno@bluewin.ch>
    Signed-off-by: Ralf Baechle <ralf@linux-mips.org>