dt-bindings: arm: brcmstb: Correct BIUCTRL node documentation

Correct the Device Tree bindings for the HIF_CPUBIUCTRL node whose
compatible string is actually brcm,bcm<chip-id>-cpu-biu-ctrl. Also
document in the binding the fallback property
("brcm,brcmstb-cpu-biu-ctrl") and update the example accordingly.

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