[POWERPC] Remove unused, undocumented #cpus property from cpus node

The #cpus property is unused and undocumented and is therefore
being removed.

Signed-off-by: Stuart Yoder <stuart.yoder@freescale.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>