PCI: Add function for parsing 'slot-power-limit-milliwatt' DT property

PCI: Add function for parsing 'slot-power-limit-milliwatt' DT property

Add function of_pci_get_slot_power_limit(), which parses the
'slot-power-limit-milliwatt' DT property, returning the value in
milliwatts and in format ready for the PCIe Slot Capabilities Register.

Link: https://lore.kernel.org/r/20220412094946.27069-4-pali@kernel.org
Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Bjorn Helgaas <bhelgaas@google.com>