MAINTAINERS: add bgmac section entry

This driver exists for years but was missing its MAINTAINERS entry.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20210107180051.1542-3-zajec5@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>