PCI: imx6: Remove LTSSM disable workaround

This causes CPU hangs when the system is reset by the watchdog, as the GPRs
aren't cleared, but the clocks are back to disabled state.

If the bootloader uses PCIe, it must take care to bring it down into a safe
state, before passing control to the Linux kernel.  This is the only way to
get a properly operating system at all times and circumstances.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>