PCI: imx6: Rename imx6_* with imx_*

Since this driver has evolved to support other i.MX SoCs such as
i.MX7/8/9, thus rename the 'imx6' prefix to 'imx' to avoid confusion.

The driver name is left unchanged to avoid breaking userspace.

Link: https://lore.kernel.org/linux-pci/[email protected]
Signed-off-by: Frank Li <[email protected]>
[kwilczynski: commit log, refactor the IMX_* macros]
Signed-off-by: Krzysztof Wilczyński <[email protected]>
Reviewed-by: Manivannan Sadhasivam <[email protected]>

Author: nxpfrankli