dt-bindings: power: document Broadcom's PMB binding

Broadcom's PMB is power controller used for disabling and enabling SoC
devices.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>