binding: PHY: Binding doc for NS2 PCIe PHYs.

Binding doc for NS2 PCIe PHYs.

Signed-off-by: Pramod Kumar <pramod.kumar@broadcom.com>
Signed-off-by: Jon Mason <jonmason@broadcom.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>