arm64: defconfig: enable bcmbca soc support

Enable CONFIG_ARCH_BCMBCA in defconfig. This config can be used to build
a basic kernel for arm64 based Broadcom Broadband SoC booting to
console.

Signed-off-by: William Zhang <william.zhang@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>