i2c: nordic: switch to phandle arrays for pinmux

Deprecate the scl-pin and sda-pin properties in the devicetree.
Provide new scl-gpios and sda-gpios properties instead.

This lets the user specify SCL and SDA like this:

   &i2c0 {
         scl-gpios = <&gpio0 1 0>;
         sda-gpios = <&gpio1 4 0>;
   };

Instead of having to use:

   &i2c0 {
         scl-pin = <1>;
         sda-pin = <36>;
   };

Provide error checking and understandable error messages for invalid
configurations.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>