ARM: dts: imx7d: Add node for PCIe PHY

This adds the PHY as a new node. The PCI-e controller node gains a
phandle property that points to it.

There isn't yet any code in the kernel that uses this device's
registers, but it will be added for a PCIe PLL erratum workaround.

Signed-off-by: Trent Piepho <tpiepho@impinj.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>