ARM: dts: imx6: tag boards that have the HW workaround for ERR006687

Add the DT property to all boards that have the hardware workaround
for erratum ERR006687 present. This allows the CPUidle driver to use
the deep idle states, even if the FEC is active.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>