net: mvpp2: add explicit sgmii-2500 support

Until now the mvpp2 driver used an extra 'phy-speed'
DT property in order to differentiate between the
SGMII and SGMII @2.5GHz. As there is a dedicated
PHY_INTERFACE_MODE_SGMII_2500 flag to mark the latter
start using it and drop the custom flag.

Signed-off-by: Marcin Wojtas <mw@semihalf.com>
Reviewed-by: Stefan Chulski <stefanc@marvell.com>
Reviewed-by: Nadav Haklai <nadavh@marvell.com>
Tested-by: Nadav Haklai <nadavh@marvell.com>

Signed-off-by: Stefan Roese <sr@denx.de>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>