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>