dt-bindings: gpu: Add ASPEED GFX bindings document

This describes the ASPEED BMC SoC's display controller.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://patchwork.freedesktop.org/patch/msgid/20190403001909.31637-2-joel@jms.id.au