RAS/AMD/FMPM: Add debugfs interface to print record entries

It is helpful to see the saved record entries during run time in
human-readable format. This is useful for testing during module
development. It can also be used by system admins to quickly and easily
see the state of the system.

Provide a sequential file in debugfs to print fields of interest from
the FRU records and their entries.

Don't fail to load the module if the debugfs interface is not available.
This is a convenience feature which does not affect other module
functionality.

The new interface reads the record entries and should hold the mutex.
Expand the mutex code comment to clarify when it should be held.

Signed-off-by: Yazen Ghannam <yazen.ghannam@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20240301143748.854090-4-yazen.ghannam@amd.com