spi: sh-msiof: Improve bindings

Documentation:
  - Add missing "interrupt-parent", "#address-cells", "#size-cells", and
    "clocks" properties,
  - Add missing default values for "num-cs", "renesas,tx-fifo-size" and
    "renesas,rx-fifo-size",
  - Add a reference to the pinctrl documentation.

Implementation:
  - As "num-cs" is marked optional, provide a sensible default.

Signed-off-by: Geert Uytterhoeven <geert+renesas@linux-m68k.org>
Acked-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Mark Brown <broonie@linaro.org>