net: stmmac: Add dwmac-intel-plat for GBE driver

Add dwmac-intel-plat to enable the stmmac driver in Intel Keem Bay.
Also add fix_mac_speed and tx_clk in order to change link speeds.
This is required as mac_speed_o is not connected in the
Intel Keem Bay SoC.

Signed-off-by: Rusaimi Amira Ruslan <rusaimi.amira.rusaimi@intel.com>
Signed-off-by: Vineetha G. Jaya Kumaran <vineetha.g.jaya.kumaran@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>