stmmac: update driver documentation

Recently many changes have been done inside the driver
so this patch updates the driver's doc for example reviewing
information for the rx and tx processes that are managed
by napi method, adding new information for missing glue-logic files
etc.

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