PCI: OF: Allow of_pci_get_max_link_speed() to be used by PCI Endpoint drivers

of_pci_get_max_link_speed() is built only if CONFIG_PCI is enabled.
Make of_pci_get_max_link_speed() to be also used by PCI Endpoint
controllers with just CONFIG_PCI_ENDPOINT enabled.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>