pci: imx: disable imx6sdl LTSSM upon driver remove

commit 6ecbe137 ("drivers: pci: imx: add imx_pcie_remove function")
attempted to resolve an issue caused by MX6QDL not having a proper
intneral PCIe core reset and thus hanging during kernel init if the
bootloader had enabled PCI.

The issue exists for IMX6Q, IMX6D, IXM6S, and IMX6DL. Fix the case for
IMX6S and IMX6DL getting missed.

This fixes IMX6S and IMX6DL with PCI enabled in U-Boot booting for
Linux v4.11+.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>