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>