Documentation: bindings: brcmstb: Document write-pairing

Document the hif-cpubiuctrl node a bit more, and add a documentation
entry for the optional "brcm,write-pairing" property.

Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Gregory Fong <gregory.0xf0@gmail.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>