checkpatch: Add check for gpiomux usage in msm board files

MSM has a board-*-gpiomux file where all the gpiomux configs reside.
Warn if a non gpiomux board file tries to add gpiomux configs.
The camera board file is an exception to this rule.

Change-Id: Ibab190dcbd7ea78e7ca150142c68c5ae881e4e06
Signed-off-by: Rohit Vaswani <rvaswani@codeaurora.org>
(cherry picked from commit 60d78bb9809e7d4d1c3dc1425cbfd9e649e87c1c)