dt-bindings: host1x: Document Memory Client resets of Host1x, GR2D and GR3D

Memory Client should be blocked before hardware reset is asserted in order
to prevent memory corruption and hanging of memory controller.

Document Memory Client resets of Host1x, GR2D and GR3D hardware units.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>