arm: socfpga: Handle phy-mode OF property for GMACs

arm: socfpga: Handle phy-mode OF property for GMACs

Thus far, the socfpga init code had hard-coded the configuration
of the ethernet PHY interface to RGMII in the ethernet registers
in sysmgr space, so PHYs connected in another modes did not work.

This patch fixes support for configurations where the ethernet PHYs
are connected over MII/GMII/RMII interfaces by parsing the phy-mode
OF property of the GMACs and configuring the ethernet registers in
sysmgr space accordingly.

Signed-off-by: Marek Vasut <marex@denx.de>
Reported-by: Denis Bakhvalov <denis.bakhvalov@nokia.com>
Cc: Dinh Nguyen <dinguyen@opensource.altera.com>