dt-bindings: phy: bindings document for common Broadcom SATA3 PHY driver

This patch:
1. Renames DT bindings document of Broadcom STB SATA3 PHY driver to
common Broadcom SATA3 PHY driver bindings document
2. Adds bindings info for NS2 SATA3 PHY

Signed-off-by: Anup Patel <anup.patel@broadcom.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>