gpu/drm: ingenic: Add support for the JZ4770

The LCD controller in the JZ4770 supports up to 720p. While there has
been many new features added since the old JZ4740, which are not yet
handled here, this driver still works fine.

v2: No change

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20191210144142.33143-6-paul@crapouillou.net
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Acked-by: Sam Ravnborg <sam@ravnborg.org>