firmware: arm_ffa: Remove unnecessary declaration of ffa_partitions_cleanup()

In order to keep the uniformity, just move the ffa_partitions_cleanup()
before it's first usage and drop the unnecessary forward declaration.

No functional change.

Tested-by: Viresh Kumar <viresh.kumar@linaro.org>
Message-Id: <20250217-ffa_updates-v3-13-bd1d9de615e7@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>