PCI/pwrctrl: Add optional slot clock for PCI slots

Add the ability to enable optional slot clock into the pwrctrl driver.
This is used to enable slot clock in split-clock topologies, where the PCIe
host/controller supply and PCIe slot supply are not provided by the same
clock. The PCIe host/controller clock should be described in the controller
node as the controller clock, while the slot clock should be described in
controller bridge/slot subnode.

Example DT snippet:

  &pcicontroller {
      clocks = <&clk_dif 0>;             /* PCIe controller clock */

      pci@0,0 {
          #address-cells = <3>;
          #size-cells = <2>;
          reg = <0x0 0x0 0x0 0x0 0x0>;
          compatible = "pciclass,0604";
          device_type = "pci";
          clocks = <&clk_dif 1>;         /* PCIe slot clock */
          vpcie3v3-supply = <&reg_3p3v>;
          ranges;
      };
  };

Example clock topology:
   ____________                    ____________
  |  PCIe host |                  | PCIe slot  |
  |            |                  |            |
  |    PCIe RX<|==================|>PCIe TX    |
  |    PCIe TX<|==================|>PCIe RX    |
  |            |                  |            |
  |   PCIe CLK<|======..  ..======|>PCIe CLK   |
  '------------'      ||  ||      '------------'
                      ||  ||
   ____________       ||  ||
  |  9FGV0441  |      ||  ||
  |            |      ||  ||
  |   CLK DIF0<|======''  ||
  |   CLK DIF1<|==========''
  |   CLK DIF2<|
  |   CLK DIF3<|
  '------------'

Immutable commit for Geert Uytterhoeven <geert+renesas@glider.be>

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Anand Moon <linux.amoon@gmail.com>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

drivers/pci/pwrctrl/slot.c

@@ -4,6 +4,7 @@
  * Author: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
  */
 
+#include <linux/clk.h>
 #include <linux/device.h>
 #include <linux/mod_devicetable.h>
 #include <linux/module.h>
@@ -30,6 +31,7 @@ static int pci_pwrctrl_slot_probe(struct platform_device *pdev)
 {
 	struct pci_pwrctrl_slot_data *slot;
 	struct device *dev = &pdev->dev;
+	struct clk *clk;
 	int ret;
 
 	slot = devm_kzalloc(dev, sizeof(*slot), GFP_KERNEL);
@@ -55,6 +57,12 @@ static int pci_pwrctrl_slot_probe(struct platform_device *pdev)
 	if (ret)
 		goto err_regulator_disable;
 
+	clk = devm_clk_get_optional_enabled(dev, NULL);
+	if (IS_ERR(clk)) {
+		return dev_err_probe(dev, PTR_ERR(clk),
+				     "Failed to enable slot clock\n");
+	}
+
 	pci_pwrctrl_init(&slot->ctx, dev);
 
 	ret = devm_pci_pwrctrl_device_set_ready(dev, &slot->ctx);