[U-Boot] drivers: pci: imx: add imx_pcie_remove function

[Soeren Moch]

> There is no dedicated reset signal wired up for the MX6QDL thus if the
> bootloader enables the link we need some special handling to get the core
> back into a state where it is safe to touch it for configuration.
>
> While there has been some special handling in the Linux kernel to do this,
> it was removed in 4.11 thus we need to do it properly in the bootloader
> and therefore without this if you enable PCI in the bootloader you
will hang
> while booting the 4.11 kernel.
>
> This puts the PCIe controller back into a safe state for the kernel driver
> before launching the kernel.
>
> Signed-off-by: Tim Harvey <tharvey at gateworks.com>

Successfully tested on a tbs2910 board.

Tested-by: Soeren Moch <smoch at web.de>