net: stmmac: Add DW XPCS specified via "pcs-handle" support

Recently the DW XPCS DT-bindings have been introduced and the DW XPCS
driver has been altered to support the DW XPCS registered as a platform
device. In order to have the DW XPCS DT-device accessed from the STMMAC
driver let's alter the STMMAC PCS-setup procedure to support the
"pcs-handle" property containing the phandle reference to the DW XPCS
device DT-node. The respective fwnode will be then passed to the
xpcs_create_fwnode() function which in its turn will create the DW XPCS
descriptor utilized in the main driver for the PCS-related setups.

Signed-off-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>