dt-bindings: cpufreq: Document support for Airoha EN7581 CPUFreq

On newer Airoha SoC, CPU Frequency is scaled indirectly with SMC commands
to ATF.

A virtual clock is exposed. This virtual clock is a get-only clock and
is used to expose the current global CPU clock. The frequency info comes
by the output of the SMC command that reports the clock in MHz.

The SMC sets the CPU clock by providing an index, this is modelled as
performance states in a power domain.

CPUs can't be individually scaled as the CPU frequency is shared across
all CPUs and is global.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>