UPSTREAM: Documentation/devicetree: Add new property to specify the max link speed

UPSTREAM: Documentation/devicetree: Add new property to specify the max link speed

Some of the host drivers have the requirement of knowing whether the
EP would never train at some link speed at all. For instance, on some
boards, the link won't train at 5 GT/s but the host driver still sacrifice
some cycle to wait for the resule of training at 5 GT/s as the host could
actually support 5 GT/s. So we could parse this new property and make the
host drivers be aware of these cases.

Change-Id: I7f557282462a7146d8d15af560001c81ccc7e1a7
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>
Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
(am from git.kernel.org/cgit/linux/kernel/git/next/linux-next.git
commit 2fa39159b6a9931d6fd82ecbe65357e0ad77e1a4)