mips: fix setup_zero_pages() prototype

setup_zero_pages() has a local declaration in a platform specific header,
but that is not seen in the file it is defined in:

arch/mips/mm/init.c:60:6: error: no previous prototype for 'setup_zero_pages' [-Werror=missing-prototypes]

Move it to the corresponding global header and include that where needed.

Link: https://lkml.kernel.org/r/20231204115710.2247097-11-arnd@kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Stephen Rothwell <sfr@rothwell.id.au>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>