dts/bindings: use const to validate #<FOO>-cells

In most cases #<FOO>-cells should be a constant.  For example in spi
controller #address-cells should be 1, and #size-cells should be 0.

Use the const attribute to specify such single known values.  Add const
value to missing bindings which have cells.

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