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>

arch/powerpc/mm/mem.c

@@ -59,6 +59,7 @@
 
 int init_bootmem_done;
 int mem_init_done;
+unsigned long memory_limit;
 
 /*
  * This is called by /dev/mem to know if a given address has to