dt-bindings: More use 'enum' instead of 'oneOf' plus 'const' entries

'enum' is equivalent to 'oneOf' with a list of 'const' entries, but 'enum'
is more concise and yields better error messages.

Fix a couple more cases which have appeared.

Cc: Rob Clark <robdclark@gmail.com>
Cc: Sean Paul <sean@poorly.run>
Cc: Mark Brown <broonie@kernel.org>
Cc: Wim Van Sebroeck <wim@linux-watchdog.org>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Jonathan Marek <jonathan@marek.ca>
Cc: Aswath Govindraju <a-govindraju@ti.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: dri-devel@lists.freedesktop.org
Cc: freedreno@lists.freedesktop.org
Cc: linux-spi@vger.kernel.org
Cc: linux-watchdog@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20210910165153.2843871-1-robh@kernel.org