This commits add documentation to the FXOS8700 params and adds generic
exclude patters to the doc check, so that when boards provide custom
FXOS8700 params they do not need to document them again. The reasoning
is that the documentation in the central place is sufficient.
In addition, a generic exclude patter is added to match FXOS8700_REG_*,
as the register names are relatively self-explaining and an
implementation detail of the driver that doesn't need public
documentation.
