arm64: Add Broadcom Set Top Box Kconfig entry point

Add an ARCH_BRCMSTB Kconfig symbol which allows us not to update the
dependencies for all STB-related drivers. Select BRCMSTB_L2_IRQ and
GENERIC_IRQ_CHIP which are required for proper functioning.

Signed-off-by: Doug Berger <opendmb@gmail.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Markus Mayer <mmayer@broadcom.com>