firmware: arm_scmi: Remove duplicate declaration of struct scmi_protocol_handle

struct scmi_protocol_handle is declared twice, let us remove the duplicate
declaration.

Link: https://lore.kernel.org/r/20210427033031.4580-1-wanjiabing@vivo.com


Reviewed-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
[sudeep.holla: minor updates to the title and the changelog]
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>