spi: sh-spi: Improve bindings

  - Add future-proof "renesas,hspi-<soctype>" compatible values,
  - Add missing "interrupt-parent", "#address-cells", and "#size-cells"
    properties,
  - Add reference to pinctrl documentation,
  - Add example bindings.

Signed-off-by: Geert Uytterhoeven <geert+renesas@linux-m68k.org>
Signed-off-by: Mark Brown <broonie@linaro.org>