stmmac: change descriptor layout

This patch completely changes the descriptor layout to improve
the whole performances due to the single read usage of the
descriptors in critical paths.

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