dt-bindings: imx6q-pcie: Add description of imx7d pcie phy

There is a separate PHY device with its own registers on imx7d.  It's
currently unused, but a PCIe erratum on imx7d will require it for the
workaround.

Signed-off-by: Trent Piepho <tpiepho@impinj.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Lucas Stach <l.stach@pengutronix.de>