net: stmmac: dwmac-meson8b: Fetch the "timing-adjustment" clock

The PRG_ETHERNET registers have a built-in timing adjustment circuit
which can provide the RX delay in RGMII mode. This is driven by an
external (to this IP, but internal to the SoC) clock input. Fetch this
clock as optional (even though it's there on all supported SoCs) since
we just learned about it and existing .dtbs don't specify it.
Reviewed-by: NAndrew Lunn <andrew@lunn.ch>
Signed-off-by: NMartin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/stmicro/stmmac/dwmac-meson8b.c

@@ -85,6 +85,7 @@ struct meson8b_dwmac {
 	phy_interface_t			phy_mode;
 	struct clk			*rgmii_tx_clk;
 	u32				tx_delay_ns;
+	struct clk			*timing_adj_clk;
 };
 
 struct meson8b_dwmac_clk_configs {
@@ -380,6 +381,13 @@ static int meson8b_dwmac_probe(struct platform_device *pdev)
 				 &dwmac->tx_delay_ns))
 		dwmac->tx_delay_ns = 2;
 
+	dwmac->timing_adj_clk = devm_clk_get_optional(dwmac->dev,
+						      "timing-adjustment");
+	if (IS_ERR(dwmac->timing_adj_clk)) {
+		ret = PTR_ERR(dwmac->timing_adj_clk);
+		goto err_remove_config_dt;
+	}
+
 	ret = meson8b_init_rgmii_tx_clk(dwmac);
 	if (ret)
 		goto err_remove_config_dt;