firmware: arm_scmi: Populate perf commands rate_limit

Arm SCMI spec. v3.2, s4.5.3.4 PERFORMANCE_DOMAIN_ATTRIBUTES
defines a per-domain rate_limit for performance requests:
"""
Rate Limit in microseconds, indicating the minimum time
required between successive requests. A value of 0
indicates that this field is not supported by the
platform. This field does not apply to FastChannels.
""""
The field is first defined in SCMI v1.0.

Add support to fetch this value and advertise it through
a rate_limit_get() callback.

Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>
Reviewed-by: Cristian Marussi <cristian.marussi@arm.com>
Acked-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>