boards: dts: nrf: Remove SoC compatible in top-level compatible

The SoC node has compatibles for the specific SoC in place, having the
same compatible at the top level is technically a conflict and the
top-level one should really just be about the board.  Remove the SoC
related compatibles at the top-level.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>