Fix section mismatch in free_initmem

WARNING: arch/arc/mm/built-in.o(.text+0x11cc): Section mismatch in reference from
the function free_initmem() to the function .init.text:mmapcode_space_init()

Add __init_refok to free_initmem() definition per upstream Linux code.

Change-Id: I5984c80cc30b6ac8b225f9fc8292d761139d90eb
diff --git a/arch/arc/mm/init.c b/arch/arc/mm/init.c
index 32d658a..174bc80 100755
--- a/arch/arc/mm/init.c
+++ b/arch/arc/mm/init.c
@@ -75,7 +75,7 @@
 	       codesize >> 10, datasize >> 10, initsize >> 10);
 }
 
-void free_initmem(void)
+void __init_refok free_initmem(void)
 {
 	extern char _init_begin, _init_end;
 	unsigned long addr;