On modern CPUs the CTRL register is read-only except bit 63 which is
the run latch control. This means it can be updated with a mtspr
rather than mfspr/mtspr.

To accomodate older CPUs (Cell at least), where there are other bits
in the register, we still do a read/modify/write on pre 2.06 CPUs.
Signed-off-by: NNicholas Piggin <npiggin@gmail.com>
[mpe: Update change log to mention 2.06 workaround]
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>