PCI: imx6: Add IMX8MM_EP and IMX8MP_EP fixed 256-byte BAR 4 in epc_features

For IMX8MM_EP and IMX8MP_EP, add fixed 256-byte BAR 4 and reserved BAR 5
in imx8m_pcie_epc_features.

Fixes: 75c2f26d ("PCI: imx6: Add i.MX PCIe EP mode support")
Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>
[bhelgaas: add details in subject]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20250708091003.2582846-3-hongxing.zhu@nxp.com