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

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>

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