scripts: edtlib: Rename Interrupt/GPIO/etc. 'specifier' field to 'data'

'Specifier' is devicetree specalese for data associated with interrupts,
GPIOs, etc., e.g. <1 2> and <3 4> in

    pwms = <&ctrl-1 1 2 &ctrl-2 3 4>;

It's probably unnecessarily confusing to call it that. Call it 'data'
instead, which is a bit more straightforward.

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