ARM: BCM63xx: Add secondary CPU PMB initialization sequence

The sequence to initialize a secondary CPU using the BCM63138 PMB is
extremely specific and represents much more code than any other on-chip
peripheral (AHCI, USB 3.0 or integrated Ethernet switch), as such we
keep that code local and utilize Device Tree to lookup all the resources
we need from the CPU device tree node.

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