ARM: sunxi: fix the core number of V3s in sunxi README

The Allwinner V3s SoC is not quad-core, but single-core.

Fix this in the README file.

Fixes: b074fede ("arm: sunxi: add support for V3s SoC")
Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>