ARM: STi: STiH415: Add ethernet support.

This patch adds support to STiH415 SOC, which has two ethernet
snps,dwmac controllers version 3.610. With this patch B2000 and B2020
boards can boot with ethernet in MII and RGMII modes.

Tested on both B2020 and B2000.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@st.com>