ARM: sunxi: Add R8 support

The R8 is a new Allwinner SoC based on the A13. While both are very
similar, there's still a few differences. Introduce a new compatible to
deal with them.

In order to have a consistent naming, instead of mentioning the Allwinner
A series as the machine name, switch to sun4i/sun5i like what is done for
the other families.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Stephen Boyd <sboyd@codeaurora.org>