dt-bindings: imx6q-pcie: Add turnoff reset for imx7d

This is documented as "required" but won't be present in old dtbs.

These resets are also present on other imx chips but right now only
imx7d implements them through the reset controller subsystem.

Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Rob Herring <robh@kernel.org>