powerpc: Add cputable entry for POWER7

Add a cputable entry for the POWER7 processor.

Also tell firmware that we know about POWER7.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Joel Schopp <jschopp@austin.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>