MIPS: mm: Use the bitmap API to allocate bitmaps

Use bitmap_zalloc() instead of hand-writing them.

It is less verbose and it improves the semantic.

While at it, turn a bitmap_clear() into an equivalent bitmap_zero(). It is
also less verbose.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>