ARM: dts: imx6qdl-sabresd: remove power-on gpio from pcie

This isn't compatible with the new binding and should be
handled via a proper regulator. It shouldn't be needed as
the driver has always ignored this property.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>