ARM: SPEAr: DT: Modify DT bindings for STMMAC

This patch modifies the DT bindings for the GMAC IP existings for the
SPEAr family. The DT bindings now additionally pass the phy mode as a
configuration parameter for the ethernet device.

Signed-off-by: Deepak Sikri <deepak.sikri@st.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>