dt-bindings: phy: Document cadence Sierra PHY bindings

Add DT binding documentation for Sierra PHY.  The PHY supports
a number of different protocols, including PCIe and USB.

The PHY lanes may be configured as single or multi-lane links.
Each link is treated as a separate sub-node.  For example, if
there are 4 lanes in total the first 2 might be configured as
a multi-lane PCIe link while the other two are single lane
USB links, and in this case there would be 3 sub-nodes.

There are two resets for the PHY block (one for APB register
access, one for the PHY link) and separate resets for each
link.  For multi-lane links, the reset corresponds to the
reset line on the master lane, the resets on other lanes
have no effect.

Signed-off-by: Alan Douglas <adouglas@cadence.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>