clk: sunxi: Add sun6i/8i display support

Add the clock type which is used by the sun6i/8i families for video display.

Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>