[POWERPC] dont allow pSeries_probe to succeed without initialising MMU

pSeries_probe can decide that we are a pseries but then fail to
initialise the MMU. If an rtas node doesnt exist, we continually fall
out of pSeries_probe_hypertas early and never get to the MMU init code.

While pseries without RTAS is an illegal combination, the way we
currently fail is a pain to track down, and can happen if your flattened
device tree code has issues (like mine did :).

With the following patch we init the MMU, come up and print some
warnings about RTAS not existing, instead of looping on 0x400 exceptions.

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