mfd: Add bcm590xx pmu DT binding

Add a DT binding for the BCM590xx PMUs. The binding inherits from
the generic regulator bindings.

Signed-off-by: Matt Porter <mporter@linaro.org>
Reviewed-by: Tim Kryger <tim.kryger@linaro.org>
Reviewed-by: Markus Mayer <markus.mayer@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>