k64f: Fix mixing GPIO and pinmux concepts

Setting the direction of a GPIO pin is not the responsibility of the
pinmux "board" initialisation. This should be left for the GPIO
utilising application.

Some macros that were only used when setting the pin direction are
removed.

Change-Id: I5b63d52446a27fe539c89f0639a8dcadf5ea9f80
Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>