stmmac: update the Doc and Version (PTP+SGMII)

This patch updates the stmmac.txt file adding information related to the PTP
and SGMII/RGMII supports.

Also the patch updates the driver version to: March_2013.

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