ARM: dts: at91: sama7g5ek: disable slew rate for GMACs non MDIO pins

Non GMAC's MDIO pins should have slew rate disabled for R(G)MII modes. Set
them accordingly in DT.

Signed-off-by: Codrin Ciubotariu <codrin.ciubotariu@microchip.com>
Tested-by: Eugen Hristev <eugen.hristev@microchip.com>
Reviewed-by: Claudiu Beznea <claudiu.beznea@microchip.com>