dt-bindings: dvfs: Add support for generic performance domains

The CLKSCREW attack [0] exposed security vulnerabilities in energy management
implementations where untrusted software had direct access to clock and
voltage hardware controls. In this attack, the malicious software was able to
place the platform into unsafe overclocked or undervolted configurations. Such
configurations then enabled the injection of predictable faults to reveal
secrets.

Many Arm-based systems used to or still use voltage regulator and clock
frameworks in the kernel. These frameworks allow callers to independently
manipulate frequency and voltage settings. Such implementations can render
systems susceptible to this form of attack.

Attacks such as CLKSCREW are now being mitigated by not having direct and
independent control of clock and voltage in the kernel and moving that
control to a trusted entity, such as the SCP firmware or secure world
firmware/software which are to perform sanity checking on the requested
performance levels, thereby preventing any attempted malicious programming.

With the advent of such an abstraction, there is a need to replace the
generic clock and regulator bindings used by such devices with a generic
performance domains bindings.

[0] https://www.usenix.org/conference/usenixsecurity17/technical-sessions/presentation/tang



Cc: Rob Herring <robh+dt@kernel.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>