ARM: dts: sun9i: Add enable-method for SMP support for the A80 SoC

Using the enable-method property for SMP support would allow future PSCI
implementations to override any in-OS support. This is better than just
matching against the machine compatible, and then having to determine
whether other methods are available or not.

This adds enable-method properties to all CPU nodes.

Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Chen-Yu Tsai <wens@csie.org>