scripts/dts/edtlib.py: add 'const' support to bindings

Add a 'const' property to bindings for any properties that are expected
to have a specifi known value.  For example, #address-cells for an I2C
bus should always be '1'.  So we can do something like the following in
the I2C bus binding:

    "#address-cells":
      type: int
      category: required
      const: 1

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>
Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>