pci: pcie-rcar-gen4: Handle always on reference clock

The clock controller driver for the device which supplies reference
clock to the PCIe core and bus may not implement .enable / .disable
callbacks in case the clock supplied by the device are always on,
in which case the clk_prepare_enable() call returns -ENOSYS. This
is the case on e.g. 9FGV0441. Handle the -ENOSYS as a valid return
value and proceed with probing.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>

drivers/pci/pci-rcar-gen4.c

@@ -451,7 +451,7 @@ static int rcar_gen4_pcie_probe(struct udevice *dev)
 		return PTR_ERR(rcar->ref_clk);
 
 	ret = clk_prepare_enable(rcar->ref_clk);
-	if (ret)
+	if (ret && ret != -ENOSYS)
 		return ret;
 
 	ret = gpio_request_by_name(dev, "reset-gpios", 0, &rcar->pe_rst,