MIPS: Make declaration for function `memory_region_available` static

Fix non-fatal warning during compilation using W=1:

arch/mips/kernel/setup.c:158:13: warning: no previous prototype for ‘memory_region_available’ [-Wmissing-prototypes]
 bool __init memory_region_available(phys_addr_t start, phys_addr_t size)
             ^~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/18175/


[jhogan@kernel.org: tweak whitespace]
Signed-off-by: James Hogan <jhogan@kernel.org>