gianfar: Add support for hibernation

Thanks to various cleanups and refactorings this is now straightforward:
convert the gianfar driver to dev_pm_ops, plus add ->restore() callback
that will fully reinitialize MAC internal registers and BDs.

Note that I kept legacy suspend/resume callbacks so that this patch
doesn't depend on PowerPC changes (i.e. dev_pm_ops support for OF
platform drivers).

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: David S. Miller <davem@davemloft.net>