drm/aspeed: Look up syscon by phandle

This scales better to multiple families of SoC. The lookup by compatible
can be removed in a future change.

The fallback path is for the ast2500 platform only. Other platforms will
be added with the new style, so they won't need fallback paths.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://patchwork.freedesktop.org/patch/msgid/20210209123734.130483-2-joel@jms.id.au