dts: nxp: kinetis: Update nxp,kinetis-gpio binding for port connection

Add a property to the nxp,kinetis-gpio binding that related the GPIO
node to the pinmux PORT node.

For the kl25z we add the pinmux nodes as well since they didn't exist.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>