devicetree: bindings: Add vendor prefix for Shenzhen Xunlong Software

Shenzhen Xunlong Software CO.,Limited is a SBC vendor, which produces
the "Orange Pi" series of SBCs.

Add a vendor prefix for it. This prefix is already used in many
Allwinner H3 boards.

Signed-off-by: Icenowy Zheng <icenowy@aosc.xyz>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>