MAINTAINERS: Add ASPEED BMC GFX DRM driver entry

This hardware is found inside ASPEED Baseboard Management Controller
(BMC) system on chips. It is called the 'SOC Display Controller' or 'GFX'.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Acked-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://patchwork.freedesktop.org/patch/msgid/20190403001909.31637-4-joel@jms.id.au