MAINTAINERS: BCM2835: Add internal Broadcom mailing list

There is a patchwork instance behind bcm-kernel-feedback-list that is
helpful to track submissions for the Broadcom ARM-SoC maintainers and
make sure there are no patches missed, add this list for the Broadcom
BCM2835 architecture.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>