ARM: dts: remove bcm11351-brt.dts

The BCM11351 BRT board will never see the light of day. Remove the BRT
dts since it is not maintainable.

Reviewed-by: Alex Elder <elder@linaro.org>
Reviewed-by: Christian Daudt <bcm@fixthebug.org>
Reviewed-by: Markus Mayer <markus.mayer@linaro.org>
Signed-off-by: Matt Porter <mporter@linaro.org>