stmmac: Add device-tree support

This patch adds support to configure the STMMAC ethernet driver via
device-tree instead of platform_data.

Currently, only the properties needed on SPEAr600 are provided. All
other properties should be added once needed on other platforms.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>