ARM: dts: imx7s-warp: correct vendor in compatible to Element14

The WaRP7 board (Wearable Development Platform) was apparently made by
Element14, not by "Warp".  Correct the  vendor in compatible to reflect
this.  The compatibles were not documented in the bindings before.

Link: https://www.element14.com/community/docs/DOC-79058


Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>