mraa: Add flags to not setup bus pinmux

mraa: Add flags to not setup bus pinmux

This is an attempt to make the mraa work on wide boards with minimal
board setup.

Most of the arm boards with latest kernel have pinmux already setup by
the driver and its really not necessary for mraa to check each pin and
icheck if the pinmux is required or not. Having this flag would cut lot
of code in board support and its possible to add some generic board
support which can make use of mraa easily.

Without this patch each board support has to have pins setup in there
bus structure.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>