dt-bindings: arm: Document the rest of i.MX28 based boards

Not all compatibles of i.MX28 based boards are documented, which
cause dtbs_check warnings. So add the missing compatibles to
fsl.yaml.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>