scripts/dts/extract: Add single extract to extract_{controller,cells}

Add the ability to extract just a single instance from extract_cells and
extract_controller.  This lets us pass in a prop_values like:

cs-gpios =  < 0x02 0x11 0x00 >, < 0x03 0x05 0x00 >;

and extract just index = 1 if we want < 0x03 0x05 0x00 >.

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