net: stmmac: support max-speed device tree property

This patch adds support to "max-speed" property which is a standard
Ethernet device tree property. max-speed specifies maximum speed
(specified in megabits per second) supported the device.

Depending on the clocking schemes some of the boards can only support
few link speeds, so having a way to limit the link speed in the mac
driver would allow such setups to work reliably.

Without this patch there is no way to tell the driver to limit the
link speed.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@st.com>
Acked-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>