drm/sun4i: Allow building on arm64

64-bit ARM SoCs from Allwinner have DE2/TCON/HDMI periphery which
is compatible to 32-bit SoCs, so allow building DRM driver for
arm64 architecture.

Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180301213442.16677-14-jernej.skrabec@siol.net