UPSTREAM: of/pci: Add helper function 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.

Change-Id: I430b05fa5fd25fe17cf1bd8b1226e460eb7dd14b
Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
(am from git.kernel.org/cgit/linux/kernel/git/next/linux-next.git
commit 9a1dc3891255afd836f355b117fd6b975d0b1eb2)