MAINTAINERS: fix broken doc refs due to yaml conversion

Several *.txt files got converted to yaml. Update their
references at MAINTAINERS file accordingly.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Acked-by: Stephen Boyd <sboyd@kernel.org>
Link: https://lore.kernel.org/r/3b58afec5195d4ea505ea9b3f74d53f7abed4e6f.1603791716.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>