dt-bindings: Add bindings for Broadcom STB DRAM Sensors

Provide bindings for the Broadcom STB DDR PHY Front End (DPFE).

Signed-off-by: Markus Mayer <mmayer@broadcom.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>