ARM: brcmstb: reintroduce SMP support

Support for SMP bringup of the B15 CPUs on Broadcom STB chips was added
in commit 4fbe66d9 but was reverted in
commit fc3e825f to address some late
review comments. This reintroduces SMP support.

Signed-off-by: Marc Carino <marc.ceeeee@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>