The VGEN_1V8 is used on SCH-28857 REV B sabresd board.
Add the according vgen3 into PCIe node.
Thus, the external OSC can keep toggling with the correct
power supply.
Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com> Acked-by: Fugang Duan <Fugang.duan@nxp.com>