Add reg-name: "dbi2", "atu" for i.MX8M PCIe Endpoint.

For i.MX8M PCIe EP, the dbi2 and atu addresses are pre-defined in the
driver. This method is not good.

In commit b7d67c61 ("PCI: imx6: Add iMX95 Endpoint (EP) support"),
Frank suggests to fetch the dbi2 and atu from DT directly. This commit is
preparation to do that for i.MX8M PCIe EP.

These changes wouldn't break driver function. When "dbi2" and "atu"
properties are present, i.MX PCIe driver would fetch the according base
addresses from DT directly. If only two reg properties are provided, i.MX
PCIe driver would fall back to the old method.

Link: https://lore.kernel.org/linux-pci/1723534943-28499-2-git-send-email-hongxing.zhu@nxp.comSigned-off-by: Richard Zhu <hongxing.zhu@nxp.com>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>