ARM: BCM63xx: Add SMP support for BCM63138

Add support for booting the secondary CPU on BCM63138, this involves:

- locating the bootlut to write the reset vector
- powering up the second CPU when we need to using the DT-supplied PMB
  references
- disabling VFP when enabled such that we can keep having SMP

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