dts: arm: nxp: Add label to pinmux ports for Kinetis SoCs

Add 'label' as a required property for nxp,kinetis-pinmux devices.  In
addition we cleanup the kw2xd node labels for the pinmux devices to
match the SoC docs and use 'port' instead of 'pinmux_'.

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