powerpc/83xx: Add power management support for MPC8315E-RDB boards

- Add nodes for PMC and GTM controllers. GTM4 can be used as a wakeup
  source;

- Add fsl,magic-packet properties to eTSEC nodes, i.e. wake-on-lan
  support. Unlike MPC8313 processors, MPC8315 can resume from deep
  sleep upon magic packet reception.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Acked-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>