ARM: sunxi: dt: Reorganize the dtsi

In the early days, the A10 and A13 shared quite some code. Nowadays it
shares less and less code, the A31 diverging even more, so it doesn't
make much sense to continue to maintain this structure, just use one
DTSI for every SoC, and that's it.

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