dt-bindings: add compatible string of Allwinner H5 Mali-450 MP4 GPU

Allwinner H5 has a Mali-450 MP4 GPU, which has a reset line like other
Allwinner SoCs with Mali Utgard, but it's a Mali-450, so it needs a new
compatible.

Add the new compatible to Mali Utgard binding document.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Rob Herring <robh@kernel.org>