MAINTAINERS: ARM: BCM63xx: Update git and mailing-lists

arm-bcm63xx.git has not been used for development, instead, all patches
have been added to stblinux.git, so reflect that. While at it, add
bcm-kernel-feedback-list@broadcom.com as a secondary list to include
submissions, and finally update LAKML to reflect that it is moderated.

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