stmmac: Add an optional device tree property "snps,burst_len"

This property define the AXI bug lenth.
Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/stmmac.txt

@@ -43,6 +43,7 @@ Optional properties:
   available this clock is used for programming the Timestamp Addend Register.
   If not passed then the system clock will be used and this is fine on some
   platforms.
+- snps,burst_len: The AXI burst lenth value of the AXI BUS MODE register.
 
 Examples:
 

drivers/net/ethernet/stmicro/stmmac/stmmac_platform.c

@@ -235,6 +235,9 @@ static int stmmac_probe_config_dt(struct platform_device *pdev,
 			of_property_read_bool(np, "snps,fixed-burst");
 		dma_cfg->mixed_burst =
 			of_property_read_bool(np, "snps,mixed-burst");
+		of_property_read_u32(np, "snps,burst_len", &dma_cfg->burst_len);
+		if (dma_cfg->burst_len < 0 || dma_cfg->burst_len > 256)
+			dma_cfg->burst_len = 0;
 	}
 	plat->force_thresh_dma_mode = of_property_read_bool(np, "snps,force_thresh_dma_mode");
 	if (plat->force_thresh_dma_mode) {