- Add bcm2835_mbox_property for generic property request, it accepts
pointer to prepared property chan message and its size, forwards
it to MBOX and copies result back
- Make all bcm2835_mbox_XXX functions that use property channel go
through bcm2835_mbox_property path. Do not accept device_t as
an argument, it's not required: all DMA operatiosn should go
through mbox device, and all API consumers should report errors
on their side.
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