mfd: dt: Add bindings for the Aspeed SoC Display Controller (GFX)

The Aspeed SoC Display Controller is presented as a syscon device to
arbitrate access by display and pinmux drivers. Video pinmux
configuration on fifth generation SoCs depends on bits in both the
System Control Unit and the Display Controller.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>