sunxi: Change the machine compatible string.

Commit 68136b10

 ("ARM: sunxi: Change device tree naming scheme for
sunxi") changed the naming scheme and the compatible strings used in the
device trees related to the sunXi platform, but forgot to change the
compatible string in the DT machine definition.

This prevents the kernel from booting on these boards.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>