MAINTAINERS: add brcmstb regex

This could probably consolidate a few file listings. And it satisfies
the spirit of the highly annoying [1] checkpatch warning for every new
file, though it sadly won't quash it.

[1] https://lkml.org/lkml/2014/12/17/24



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