i2c: pca954x: add device tree binding

No longer rely on the implicit matching with the i2c device name, use
an explicit compatible string instead.

Keep a direct pointer to the chip description instead of an index into
the chip description array.

Reviewed-by: Wolfram Sang <wsa@the-dreams.de>
Signed-off-by: Peter Rosin <peda@axentia.se>