MAINTAINERS: Update BCM63XX entry

There are more files being supported by the BCM63XX than simply
"arch/arm/mach-bcm/bcm63xx.c" and "arch/arm/include/debug/bcm63xx.S".
Add a regex of "bcm63xx" to catch all the other files that are out
there.

Signed-off-by: Jon Mason <jon.mason@broadcom.com>
Acked-by: Scott Branden <scott.branden@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>