powerpc: Fix bug arising from having multiple memory_limit variables

We had a static memory_limit in prom.c, and then another one defined
in setup_64.c and used in numa.c, which resulted in the kernel crashing
when mem=xxx was given on the command line.  This puts the declaration
in system.h and the definition in mem.c.  This also moves the
definition of tce_alloc_start/end out of setup_64.c.

Signed-off-by: Paul Mackerras <paulus@samba.org>

include/asm-powerpc/system.h

@@ -179,6 +179,7 @@ extern struct task_struct *_switch(struct thread_struct *prev,
 
 extern unsigned int rtas_data;
 extern int mem_init_done;	/* set on boot once kmalloc can be called */
+extern unsigned long memory_limit;
 
 extern int powersave_nap;	/* set if nap mode can be used in idle loop */