ARM: dts: sunxi: Add num-cs for A20 spi nodes

The spi0 controller on the A20 have up to 4 CS (Chip Select) while the
others three only have 1.
Add the num-cs property to each node.
The current driver doesn't read this property but this is useful for
downstream user of DTS (FreeBSD for example).

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