ARM: dts: aspeed: Add GFX node to AST2600

The GFX device is present in the AST2600 SoC.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Tommy Haung <tommy_huang@aspeedtech.com>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Link: https://lore.kernel.org/r/20220302024930.18758-2-tommy_huang@aspeedtech.com


Signed-off-by: Joel Stanley <joel@jms.id.au>