rust: make platform device implement the Send and Sync traits

Currently, platform device holds only a pointer, which is safe to be
used from any thread. This is useful for devices that are not probed,
such as virtual devices.

[Lyude: cherry-picked from 9a6a07a9828ea840194c8428d45cd54d5c386265
https://github.com/mairacanal/linux/tree/vgem/wip-dma

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Signed-off-by: Maíra Canal <mcanal@igalia.com>