dt-bindings: phy: Add documentation for NSP SATA PHY

Add documentation for SATA PHY available in Broadcom's
NSP SoC.

Signed-off-by: Yendapally Reddy Dhananjaya Reddy <[e-mail]>
Acked-by: Florian Fainelli <[e-mail]>
Signed-off-by: Tejun Heo <[e-mail]>

Documentation/devicetree/bindings/phy/brcm-sata-phy.txt

···
      "brcm,bcm7425-sata-phy"
      "brcm,bcm7445-sata-phy"
      "brcm,iproc-ns2-sata-phy"
+     "brcm,iproc-nsp-sata-phy"
      "brcm,phy-sata3"
 - address-cells: should be 1
 - size-cells: should be 0
···
 
 Sub-nodes optional properties:
 - brcm,enable-ssc: use spread spectrum clocking (SSC) on this port
-     This property is not applicable for "brcm,iproc-ns2-sata-phy".
+     This property is not applicable for "brcm,iproc-ns2-sata-phy" and
+     "brcm,iproc-nsp-sata-phy".
 
 Example:
 	sata-phy@f0458100 {