ARM: bcm2835: fix typos in DT binding documentation

The compatible value shouldn't include ".txt", which was probably cut
and paste from the filename.

Reported-by: Domenico Andreoli <cavokz@gmail.com>
Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Olof Johansson <olof@lixom.net>