net: stmmac: add fix_mac_speed support for socfpga

This patch adds fix_mac_speed() support for
Altera socfpga Ethernet controller. Emac splitter is a
soft IP core in FPGA system that converts GMII interface from
Synopsys mac to RGMII/SGMII interface. This splitter core is
an optional IP if user would like to use RGMII/SGMII
interface in their system. Software needs to update a register
in splitter core when there is speed change.

Signed-off-by: Ley Foon Tan <lftan@altera.com>
Signed-off-by: David S. Miller <davem@davemloft.net>