arm64: defconfig: Enable ARCH_BRCMSTB

ARCH_BRCMSTB was one of the few platforms that were not enabled by
default, since we are now adding more and more drivers relevant to that
platform get some proper build coverage.

Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>