board: freescale: Refactor NXP common mux code

Refactors similar mux code from multiple NXP boards into a common location,
and allows it to be disabled in config.

New config: CONFIG_FSL_USE_PCA9547_MUX to enable PCA9547 mux functionality.

Signed-off-by: Stephen Carlson <stcarlso@linux.microsoft.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>