powerpc/pseries: new character device(s) for RTAS functions

This is a proposal for adding chardev-based access to a select subset
of RTAS functions on the pseries platform.

The problem: important platform features are enabled on Linux VMs
through the powerpc-specific rtas() syscall in combination with
writeable mappings of /dev/mem. In typical usage, this is encapsulated
behind APIs provided by the librtas library. This paradigm is
incompatible with lockdown, which prohibits /dev/mem access.

The solution I'm working on is to add a small pseries-specific
"driver" for each functional area, exposing the relevant features to
user space in ways that are compatible with lockdown. In most of these
areas, I believe it's possible to change librtas to prefer the new
chardev interfaces without disrupting existing users.

I've broken down the affected functions into the following areas and
priorities:

High priority:
* VPD retrieval.
* System parameters: retrieval and update.

Medium priority:
* Platform dump retrieval.
* Light path diagnostics (get/set-dynamic-indicator,
  get-dynamic-sensor-state, get-indices).

Low priority (may never happen):
* Error injection: would have to be carefully restricted.
* Physical attestation: no known users.
* LPAR perftools: no known users.

Out of scope:
* DLPAR (configure-connector et al): involves device tree updates
  which must be handled entirely in-kernel for lockdown. This is the
  object of a separate effort.

See ibm-power-utilities/librtas#29 for more
details.

In this initial series, I've included just a driver for VPD
retrieval. Clients use ioctl() to obtain a file descriptor-based
handle for the VPD they want. I think this could be a good model
for the other areas too, but I'd like to get opinions on it.

For reference, I floated a different approach for system parameters
here:

https://lore.kernel.org/linuxppc-dev/[email protected]/

--- b4-submit-tracking ---
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    "change-id": "20230817-papr-sys_rtas-vs-lockdown-5c54505db792",
    "prefixes": [
      "RFC"
    ]
  }
}