power: reset: Default POWER_RESET_BRCMSTB to BMIPS_GENERIC

On Broadcom MIPS STB platforms, BMIPS_GENERIC is the Kconfig symbol that
is used, make this reboot driver default to that value to make sure we
can reboot a system properly.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>