driver:net:stmmac: Disable DMA store and forward mode if platform data force_thresh_dma_mode is set.

Some synopsys ip implementation doesn't support DMA store and forward mode,
such as BF60x. So, set force_thresh_dma_mode to use DMA thresholds only.
Update document and devicetree as well.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Acked-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/stmmac.h b/include/linux/stmmac.h
index 9e495d31..bb5deb0f 100644
--- a/include/linux/stmmac.h
+++ b/include/linux/stmmac.h
@@ -108,6 +108,7 @@
 	int bugged_jumbo;
 	int pmt;
 	int force_sf_dma_mode;
+	int force_thresh_dma_mode;
 	int riwt_off;
 	void (*fix_mac_speed)(void *priv, unsigned int speed);
 	void (*bus_setup)(void __iomem *ioaddr);