stmmac: add mixed burst for DMA

In mixed burst (MB) mode, the AHB master always initiates
the bursts with fixed-size when the DMA requests transfers
of size less than or equal to 16 beats.
This patch adds the MB support and the flag that can be
passed from the platform to select it.
MB mode can also give some benefits in terms of performances
on some platforms.

v2: fixed Coding Style

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