power: reset: Add reboot driver for brcmstb

Add support for reboot functionality on boards with ARM-based
Broadcom STB chipsets. Make it built-in by default for ARCH_BRCMSTB,
but allow it to be configurable under COMPILE_TEST.

Signed-off-by: Marc Carino <marc.ceeeee@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>