Convert the Arm Utgard GPU binding to DT schema format.

'allwinner,sun8i-a23-mali' compatible was not documented, so add it.

The 'clocks' property is now required. This simplifies the schema as
effectively all the users require 'clocks' already and the upstream
driver requires clocks.
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Acked-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Rob Herring <robh@kernel.org>