drm/sun4i: Allow building the driver on RISC-V

Allwinner D1 is a RISC-V SoC which contains a DE 2.0 engine. Let's
remove the dependency on a specific CPU architecture, so the driver can
be built wherever ARCH_SUNXI is selected.

Acked-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20220424162633.12369-6-samuel@sholland.org