spi: Improve DT binding documentation

  - Add missing verbs and periods,
  - Add spaces before opening parentheses,
  - Align list layout,
  - Correct grammar,
  - Move "cs-gpios" from the required to the optional properties
    section,
  - Remove spaces before tabs.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Mark Brown <broonie@kernel.org>