ARM: dts: aspeed-g5: Add SCU phandle to GFX node

In v5.13 the DRM driver gained support for using a phandle to the SCU,
instead of matching on the scu compatible.

Link: https://lore.kernel.org/r/20210603064536.165297-1-joel@jms.id.au


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