Improve the bindings and make them more usable:

- Pick in spi-cpha and spi-cpol from the SPI node parent,
  this will specify that we are "type 3" in the device tree
  rather than hardcoding it in the operating system.
- Drop the u32 ref from the SPI frequency: comes in from
  the SPI host bindings.
- Make spi-cpha, spi-cpol and port compulsory.
- Update the example with a real-world SPI controller,
  spi-gpio.

Cc: Noralf Trønnes <noralf@tronnes.org>
Cc: devicetree@vger.kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210701213618.3818821-1-linus.walleij@linaro.org