of/pci: Add of_pci_get_max_link_speed() to parse max-link-speed from DT

This new helper function could be used by host drivers to get the limitaion
of max link speed provided by DT.  If the property isn't assigned or is
invalid, it will return -EINVAL to the caller.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rob Herring <robh@kernel.org>