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 <[e-mail]>
Cc: Ralf Baechle <[e-mail]>
Cc: [e-mail]
Patchwork: https://patchwork.linux-mips.org/patch/18175/
[[e-mail]: tweak whitespace]
Signed-off-by: James Hogan <[e-mail]>

arch/mips/kernel/setup.c

···
 	add_memory_region(start, size, BOOT_MEM_RAM);
 }
 
-bool __init memory_region_available(phys_addr_t start, phys_addr_t size)
+static bool __init __maybe_unused memory_region_available(phys_addr_t start,
+							  phys_addr_t size)
 {
 	int i;
 	bool in_ram = false, free = true;