arm: bcmbca: Add BCMBCA sub platforms

Create new CORTEXA7, CORTEXA9 and BRAHMAB15 BCMBCA sub platform configs to
enable fine-grained selection for each type of ARMv7 SoC.

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