spi: spi-gpio: Augment device tree bindings

After we augmented the core to handle "gpio-sck"/"sck-gpios",
"gpio-mosi"/"mosi-gpios", "gpio-miso"/"miso-gpios" alike,
deprecate the old binding and put the strict modern and
recommended binding practice into place as the default for
GPIO-based SPI.

This reflects the similar change in I2C:
commit 7d29f509
("dt-bindings: i2c: i2c-gpio: Add support for named gpios")

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>