phy: lantiq: rcu-usb2: wait after clock enable

Commit 65dc2e725286 ("usb: dwc2: Update Core Reset programming flow.")
revealed that the phy isn't ready immediately after enabling it's
clocks. The dwc2_check_core_version() fails and the dwc2 usb driver
errors out.

Add a short delay to let the phy get up and running. There isn't any
documentation how much time is required, the value was chosen based on
tests.

Signed-off-by: Mathias Kresin <[e-mail]>
Acked-by: Hauke Mehrtens <[e-mail]>
Acked-by: Martin Blumenstingl <[e-mail]>
Cc: <[e-mail]> # v5.7+
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Vinod Koul <[e-mail]>

drivers/phy/lantiq/phy-lantiq-rcu-usb2.c

···
 	reset_control_deassert(priv->phy_reset);
 
 	ret = clk_prepare_enable(priv->phy_gate_clk);
-	if (ret)
+	if (ret) {
 		dev_err(dev, "failed to enable PHY gate\n");
+		return ret;
+	}
+
+	/*
+	 * at least the xrx200 usb2 phy requires some extra time to be
+	 * operational after enabling the clock
+	 */
+	usleep_range(100, 200);
 
 	return ret;
 }