dts: bindings: ps2: Replace 'should be 1/0' comments with 'const: 1/0'

Check with 'const:' that #address-cells is 1 and #size-cells is 0. That
way other values will get flagged by edtlib.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>