dt-bindings: arm: fsl: Add i.MX50 based boards

fsl,imx50-evk has been used in a devicetree for a while. kobo,aura will
be used soon.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>