The sh_eth driver has come up with an issue where the runtime_pm
code suspends it during the probe() method due to the network
device registration re-calling into the driver.

Add a note about this into the documentation.
Signed-off-by: NBen Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: NBen Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: NRafael J. Wysocki <rafael.j.wysocki@intel.com>