MAINTAINERS: Update BCM7XXX entry with additional patterns

Broadcom STB systems use the bcm7038 pattern as well as the bcm7120
pattern for some of its drivers, add those to the existing entry.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20211028163756.4014059-1-f.fainelli@gmail.com'
Signed-off-by: Arnd Bergmann <arnd@arndb.de>