Use the existence of RTAS device tree node to determine if
/proc/rtas. /proc/ppc64/rtas are to be created.  Using machine type
is not reliable (i.e. Maple-like machines may have RTAS).
Signed-off-by: Michal Ostrowski <mostrows@watson.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>