device property: Introduce fwnode_get_phy_mode()

Until now there were two almost identical functions for
obtaining network PHY mode - of_get_phy_mode() and,
more generic, device_get_phy_mode(). However it is not uncommon,
that the network interface is represented as a child
of the actual controller, hence it is not associated
directly to any struct device, required by the latter
routine.

This commit allows for getting the PHY mode for
children nodes in the ACPI world by introducing a new function -
fwnode_get_phy_mode(). This commit also changes
device_get_phy_mode() routine to be its wrapper, in order
to prevent unnecessary duplication.

Signed-off-by: Marcin Wojtas <mw@semihalf.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>