MAINTAINERS: extend BCM5301X ARM ARCHITECTURE files

This patch extends the existing entry for the
"BROADCOM BCM5301X ARM ARCHITECTURE" to include
files belonging to the BCM5301X.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
Acked-by: Scott Branden <scott.branden@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>