frdm_k64f: Change PTC16-17 pinmux to select UART signals

These pins are routed to the arduino header, and configuring them for
UART signals allows us to use the frdm_k64f with a frdm_kw40z shield
board for bluetooth.

Change-Id: Ie30916409844b1dc1c6e1280d5a755a6dc42e418
Signed-off-by: Maureen Helm <maureen.helm@nxp.com>