stmmac: update the driver Documentation and add EEE

This patch updates the stmmac's documentation adding
some missing files in the section used to describe the
internal driver's structure.

Also the patch adds a new section to describe the EEE support.

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