mvebu: Turris Omnia: use SFP module, if present

mvebu: Turris Omnia: use SFP module, if present

Follow the recommendations stated in the Turris Omnia DTS for eth2:

"In case SFP module is present, U-Boot has to enable the sfp node above,
remove phy-handle property, and add managed = "in-band-status" property."

The boot script is written in a way, that it works for all U-Boot
versions deployed by the vendor so far (2015.10-rc2, 2019.07).

Reviewed-by: Noci <noci@noci.work>
Reviewed-by: Justin van Steijn <jvs@fsfe.org>
Reviewed-by: Nico Rikken <nico@nicorikken.eu>
Reviewed-by: Julius Schwartzenberg <julius.schwartzenberg@gmail.com>
Signed-off-by: Klaus Kudielka <klaus.kudielka@gmail.com>
(cherry picked from commit 23d2690e5a5410576b587bc96e2c2cf5fc693927)