PCI: Add debugfs support for exposing PTM context

Precision Time Management (PTM) mechanism defined in PCIe spec r6.0,
sec 6.21 allows precise coordination of timing information across multiple
components in a PCIe hierarchy with independent local time clocks.

PCI core already supports enabling PTM in the root port and endpoint
devices through PTM Extended Capability registers. But the PTM context
supported by the PTM capable components such as Root Complex (RC) and
Endpoint (EP) controllers were not exposed as of now. Part of the reason is
that the spec doesn't define how the context information is exposed to the
software and left it to the vendor implementation. So there is no
standardized way to get access to the context information and each vendor
have defined their own way.

This commit adds debugfs support to expose the PTM context to userspace
from both PCIe RC and EP controllers. Since the context information is
exposed in a vendor specific way, the debugfs interface allows the
controller drivers to implement callbacks for each attribute, to be called
by the generic PTM driver.

The Controller drivers are expected to call pcie_ptm_create_debugfs() to
create the debugfs attributes for the PTM context and call
pcie_ptm_destroy_debugfs() to destroy them. The drivers should also
populate the relevant callbacks in the 'struct pcie_ptm_ops' structure
based on the controller implementation.

Below PTM context are exposed through debugfs:

PCIe RC
=======

1. PTM Local clock
2. PTM T2 timestamp
3. PTM T3 timestamp
4. PTM Context valid

PCIe EP
=======

1. PTM Local clock
2. PTM T1 timestamp
3. PTM T4 timestamp
4. PTM Master clock
5. PTM Context update

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
[kwilczynski: fix overflow issue reported by Dan Carpenter from
https://lore.kernel.org/linux-pci/b41c1754-c6b7-4805-9f14-7c643d6c5304@suswa.mountain

]
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Link: https://patch.msgid.link/20250505-pcie-ptm-v4-1-02d26d51400b@linaro.org