scripts: dts: Output paths relative to $ZEPHYR_BASE

Requested by Marc Herbert. Makes the output deterministic as long as all
binding directories are within $ZEPHYR_BASE (and a bit less spammy too).

Example output for header:

Before:

    /*  Directories with bindings: /home/ulf/z/z/dts/bindings  */
    ...
    /*  Binding (...): /home/ulf/.../arm,v7m-nvic.yaml  */

After:

    /*  Directories with bindings: $ZEPHYR_BASE/dts/bindings  */
    ...
    /*  Binding (...): $ZEPHYR_BASE/dts/.../arm,v7m-nvic.yaml  */

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