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/20240729-pci2_upstream-v8-3-b68ee5ef2b4d@nxp.comSigned-off-by: Frank Li <Frank.Li@nxp.com>
[kwilczynski: commit log, refactor the IMX_* macros]
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>