PCI/pwrctrl: Add pwrctrl driver for PCI slots

PCI/pwrctrl: Add pwrctrl driver for PCI slots

This driver is used to control the power state of the devices attached to
the PCI slots. Currently, it controls the voltage rails of the PCI slots
defined in the devicetree node of the root port.

The voltage rails for PCI slots are documented in the DT-schema:

  https://github.com/devicetree-org/dt-schema/blob/v2024.11/dtschema/schemas/pci/pci-bus-common.yaml#L153

Since this driver has to work with different kind of slots (PCIe
x1/x4/x8/x16, Mini PCIe, PCI, etc.), the driver is thus using the
of_regulator_bulk_get_all() API to obtain the voltage regulators defined
in the DT node, instead of hardcoding them.

As such, the DT node of the root port should define the relevant supply
properties corresponding to the voltage rails of the PCI slot.

Tested-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Link: https://lore.kernel.org/r/20250116-pci-pwrctrl-slot-v3-5-827473c8fbf4@linaro.org
[kwilczynski: commit log]
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>