ravb: add sleep PM suspend/resume support

The interface would not function after the system had been woken up
after have been suspended (echo mem > /sys/power/state) cycle. The
reason for this is that all device registers have been reset to its
default values. This patch adds sleep suspend and resume functions that
detached the interface at suspend and restore the registers and reattach
the interface at resume.

Only the registers that are only configured at probe time needs to be
explicitly restored by the resume handler. All other registers are
reconfigured by either reopening the device in the resume handler (if
the device was running when the system was suspended) or when the
interface is opened by a user at a later time.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: David S. Miller <davem@davemloft.net>