Add register regions for the second lane of dual-lane nodes.
This additional specification is needed so that the driver can stop
reaching beyond the tx and rx register allocations to get at the
second lane registers in a dual-lane PHY.

While in there, document #clock-cells as optional for PHYs that don't
provide a pipe clock. Also, document the pcs_misc register region, which
was being quietly supplied and used.
Signed-off-by: Evan Green <evgreen@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>