Convert the Arm Midgard GPU binding to DT schema format.

The 'clocks' property is now required. This simplifies the schema as
effectively all the users require 'clocks' already and the upstream
driver requires at least one clock.
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>