Forthcoming IBM machines will have a "ibm,pa-features" property on CPU
nodes, that contains bits indicating which optional architecture
features are implemented by the CPU.  This adds code to use the
property, if present, to update our CPU feature bitmaps.  Note that
this means we can both set and clear feature bits based on what
the firmware tells us.

This is based on a patch by Will Schmidt <willschm@us.ibm.com>.
Signed-off-by: Paul Mackerras <paulus@samba.org>