ARM: sun7i: add arch timer node

The Allwinner A20 SoC is built around a pair of Cortex-A7 cores,
which have the usual generic timers. Report this in the DT.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>