firmware: arm_scmi: Replace the use of of_node_put() to __free(device_node)

Use __free for device_node values, and thus drop calls to of_node_put.

The goal is simplify of_node reference cleanup by using this scope-based
of_node_put() cleanup to simplify function exit handling. When using __free
a resource is allocated within a block, it is automatically freed at the
end of the block.

This cleanup aligns well with the recent change in shmem.c to use __free
instead of explicit of_node_put() calls.

Message-Id: <20240827143838.1465913-3-sudeep.holla@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>