dts: bindings: nxp,kinetis-pinmux: Update binding for pin data

Update the binding for how we represent pin data to support child
nodes with a nxp,kinetis-pins property that has the pin data.

For example:

      UART0_CTS_PTA0: uart0_cts_pta0 {
           nxp,kinetis-port-pins = < 0 2 >;
      };

Also pull in pincfg-node.yaml to get various common pin flags like
"bias-pull-up", "drive-open-drain", etc.

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