dt-bindings: gpu: mali-midgard: Add H6 mali gpu compatible

This add the H6 mali compatible in the dt-bindings to later support
specific implementation.

Signed-off-by: Clément Péron <peron.clem@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190521161102.29620-5-peron.clem@gmail.com