mfd: Add bcm590xx pmu driver

Add a driver for the BCM590xx PMU multi-function devices. The driver
initially supports regmap initialization and instantiation of the
voltage regulator device function of the PMU.

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>