firmware: arm_scmi: Add Powercap protocol enable support

SCMI Powercap protocol v3.2 supports disabling the powercap on a zone
by zone basis by providing a zero valued powercap.

Expose new operations to enable/disable powercapping on a per-zone base.

Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
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v1 --> v2
- simplified enable requested checks