PCI: host-generic: Rename and export gen_pci_init() for PCIe controller drivers

PCI: host-generic: Rename and export gen_pci_init() for PCIe controller drivers

Rename gen_pci_init() API as pci_host_common_ecam_create() and export it to
allow the PCIe controller drivers to create and configure the ECAM region.

Note that this API should only used by the drivers managing the drvdata on
their own. Rest should continue using pci_host_common_init() API.

Signed-off-by: Mayank Rana <mayank.rana@oss.qualcomm.com>
[mani: commit message rewording]
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Link: https://patch.msgid.link/20250616224259.3549811-3-mayank.rana@oss.qualcomm.com