PCI/AER: Move CXL RCH error handling to aer_cxl_rch.c

PCI/AER: Move CXL RCH error handling to aer_cxl_rch.c

The Restricted CXL Host (RCH) AER error handling logic currently resides
in the AER driver file, aer.c. CXL specific changes conditionally compiled
using #ifdefs.

Improve the AER driver maintainability by separating the RCH specific logic
from the AER driver's core functionality and removing the ifdefs. Introduce
drivers/pci/pcie/aer_cxl_rch.c for moving the RCH AER logic into. Conditionally
compile the file using the CONFIG_CXL_RCH_RAS Kconfig.

Move the CXL logic into the new file but leave CXL helper function
is_internal_error() in aer.c for now as it will be moved in future patch
for CXL Virtual Hierarchy handling.

To maintain compilation after the move other changes are required. Change
cxl_rch_handle_error(), cxl_rch_enable_rcec(), and is_internal_error() to
be non-static inorder for accessing from the AER driver.

Update the new file with the SPDX and 2023 AMD copyright notations because
the RCH bits were initially contributed in 2023 by AMD. See commit:
commit 0a867568bb0d ("PCI/AER: Forward RCH downstream port-detected errors to the CXL.mem dev handler")

Signed-off-by: Terry Bowman <terry.bowman@amd.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Reviewed-by: Ben Cheatham <benjamin.cheatham@amd.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Link: https://patch.msgid.link/20260114182055.46029-12-terry.bowman@amd.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>