dt-bindings: ARM: brcmstb: Update Broadcom STB Power Management binding

Update the Broadcom STB Power Management binding document with new
compatible strings for the DDR PHY and memory controller found on newer
chips.

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