scripts: dts: Format multi-line comments nicely

Multi-line comments were stuck as-is between /* and */ in the generated
header, which looks ugly and confusing e.g. for multi-line
binding/property descriptions.

Use this format for multi-line comments in the header instead:

    /*
     * First line
     * Second line
     *
     * Line after blank line
     */

Also clean up the output a bit by turning some things that were separate
comments into a single multi-line comment. Add some air and reduce line
lengths too.

Before:

    /*  Generated by gen_defines.py  */
    /*  DTS input file: hifive1.dts.pre.tmp  */
    /*  Directories with bindings: $ZEPHYR_BASE/dts/bindings  */

    /*  Devicetree node: /cpus/cpu@0/interrupt-controller  */
    /*  Binding (compatible = riscv,cpu-intc): $ZEPHYR_BASE/... */
    /*  Binding description: This binding describes the RISC-V ...

    Some extra lines
    for testing  */
    #define DT_INST_0_RISCV_CPU_INTC                    1

After:

    /*
     * Generated by gen_defines.py
     *
     * DTS input file:
     *   hifive1.dts.pre.tmp
     *
     * Directories with bindings:
     *   $ZEPHYR_BASE/dts/bindings
     */

    /*
     * Devicetree node:
     *   /cpus/cpu@0/interrupt-controller
     *
     * Binding (compatible = riscv,cpu-intc):
     *   $ZEPHYR_BASE/dts/bindings/interrupt-controller/...
     *
     * Description:
     *   This binding describes the RISC-V CPU Interrupt Controller
     *
     *   Some extra lines
     *   for testing
     */
    #define DT_INST_0_RISCV_CPU_INTC                    1

Also tweak Node.description and Property.description in edtlib to be
strip()ed instead of rstrip()ed. There's probably no reason to
preserving leading whitespace in them either.

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