firmware: arm_scmi: Refactor reset domain handling

firmware: arm_scmi: Refactor reset domain handling

Introduce scmi_reset_domain_lookup() to centralize domain ID validation
and unify error reporting behaviour across the SCMI reset protocol.

All reset domain operations are updated to use the new helper, removing
duplicated validation logic and ensuring consistent handling of invalid
domain IDs and lookup failures. This simplifies the internal flow and
improves robustness of the reset protocol implementation.

Suggested-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Artem Shimko <a.shimko.dev@gmail.com>
Message-Id: <20251123163557.230530-1-a.shimko.dev@gmail.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>