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 <yendapally.reddy@broadcom.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>