dt-bindings: arm: Add Y Soft IOTA Draco, Hydra and Ursa boards

These are i.MX6DualLite/Solo based single board computers used
in various Y Soft products.

Cc: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Michal Vokáč <michal.vokac@ysoft.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>