soc: bcm: add PM driver for Broadcom's PMB

PMB originally comes from BCM63138 but can be also found on many other
chipsets (e.g. BCM4908). It's needed to power on and off SoC blocks like
PCIe, SATA, USB.

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