MAINTAINERS: Add entry for Broadcom BDC driver

The Broadcom BDC driver did not have a MAINTAINERS entry which made it
escape review from Al and myself, add an entry so the relevant mailing
lists and people are copied.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>