ARM: BCM63xx: Add Broadcom BCM63xx PMB controller helpers

This patch adds both common register definitions and helper functions
used to issue read/write commands to the Broadcom BCM63xx PMB controller
which is used to power on and release from reset internal on-chip
peripherals such as the integrated Ethernet switch, AHCI, USB, as well
as the secondary CPU core.

This is going to be utilized by the BCM63138 SMP code, as well as by the
BCM63138 reset controller later.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>