PCI: keystone: Add support to build as a loadable module

PCI: keystone: Add support to build as a loadable module

The 'pci-keystone.c' driver is the application/glue/wrapper driver for the
Designware PCIe Controllers on TI SoCs. Now that all of the helper APIs
that the 'pci-keystone.c' driver depends upon have been exported for use,
enable support to build the driver as a loadable module.

When building the driver as a module, the functions marked by the '__init'
keyword may be invoked after the init memory has been freed by the kernel.
This results will result in an exception of the form:

  Unable to handle kernel paging request at virtual address ...
  Mem abort info:
  ...
  pc : ks_pcie_host_init+0x0/0x540
  lr : dw_pcie_host_init+0x170/0x498
  ...
  ks_pcie_host_init+0x0/0x540 (P)
  ks_pcie_probe+0x728/0x84c
  platform_probe+0x5c/0x98
  really_probe+0xbc/0x29c
  __driver_probe_device+0x78/0x12c
  driver_probe_device+0xd8/0x15c

To address this, introduce a new function namely 'ks_pcie_init()' to
register the 'fault handler' while removing the '__init' keyword from
existing functions.

Note that hook_fault_code() is defined as '__init' function. Since the init
functions should never be called during runtime (after init memory freeing
stage), the driver is made as a built-in if CONFIG_ARM (where
hook_fault_code() is used) is selected.

Signed-off-by: Siddharth Vadapalli <s-vadapalli@ti.com>
[mani: added a note about hook_fault_code()]
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/20251029080547.1253757-5-s-vadapalli@ti.com