Documentation: Extend the real-time hardware bits with some firmware bits

I have been reviewing how OP‑TEE is implemented and how secure‑world
invocations behave. The goal was to determine whether an OP‑TEE call can
delay the Linux side and introduce latency depending on the time spent
in the secure world.

Similar latency effects are already known for EFI runtime services, but
this was not documented. To mitigate the impact, EFI runtime invocations
can be restricted to specific CPUs so that real‑time workloads on other
CPUs remain unaffected. This mechanism, however, is only described in
the commit that introduced it.

This change adds a firmware section that documents these behaviours
explicitly. It highlights cases where firmware can delay the kernel,
information that may be unfamiliar to some users and surprising-or
concerning-to others.

Assisted-by: Microsoft-Copilot
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20260718175041.QXn9iOFK@linutronix.de>

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