[POWERPC] Remove spinlock from struct cpu_purr_data

cpu_purr_data is a per-cpu array used to account for stolen time on
partitioned systems.  It used to be the case that cpus accessed each
others' cpu_purr_data, so each entry was protected by a spinlock.

However, the code was reworked ("Simplify stolen time calculation")
with the result that each cpu accesses its own cpu_purr_data and not
those of other cpus.  This means we can get rid of the spinlock as
long as we're careful to disable interrupts when accessing
cpu_purr_data in process context.

Signed-off-by: Nathan Lynch <ntl@pobox.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>