rockchip: clk: rk3368: handle clk_enable requests for GMAC

Since commit ba1f96672522 ("net: designware: add clock support"), the
designware GMAC driver enables all referenced clocks. While this is a
no-op for the RK3368 during boot-up (reset behaviour has all the clock
gates open anyway), we still need to handle the clock-ids passed in
the enable op of the clock-driver and return a success.

This change extends the RK3368 clk driver to:
(a) provide a enable op
(b) signals success to the caller when the clocks for the GMAC are
    enabled (no actual action is necessary as the gates are open
    after reset)

References: commit ba1f96672522 ("net: designware: add clock support")
Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Tested-by: Klaus Goger <klaus.goger@theobroma-systems.com>

drivers/clk/rockchip/clk_rk3368.c

@@ -568,12 +568,31 @@ static int __maybe_unused rk3368_clk_set_parent(struct clk *clk, struct clk *par
 	return -ENOENT;
 }
 
+static int rk3368_clk_enable(struct clk *clk)
+{
+	switch (clk->id) {
+	case SCLK_MAC:
+	case SCLK_MAC_RX:
+	case SCLK_MAC_TX:
+	case SCLK_MACREF:
+	case SCLK_MACREF_OUT:
+	case ACLK_GMAC:
+	case PCLK_GMAC:
+		/* Required to successfully probe the Designware GMAC driver */
+		return 0;
+	}
+
+	debug("%s: unsupported clk %ld\n", __func__, clk->id);
+	return -ENOENT;
+}
+
 static struct clk_ops rk3368_clk_ops = {
 	.get_rate = rk3368_clk_get_rate,
 	.set_rate = rk3368_clk_set_rate,
 #if CONFIG_IS_ENABLED(OF_CONTROL) && !CONFIG_IS_ENABLED(OF_PLATDATA)
 	.set_parent = rk3368_clk_set_parent,
 #endif
+	.enable = rk3368_clk_enable,
 };
 
 static int rk3368_clk_probe(struct udevice *dev)