MAINTAINERS: Add Broadcom BCMBCA entry

Add maintainers, SCM tree, files and directories for the new BCMBCA
arch. Only add 47622 for this change and will update in the future
when more SoCs are supported.

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