Basic beagleplay greybus demo docs

See merge request docs/docs.beagleboard.io!61

Signed-off-by: Ayush Singh <ayushdevel1325@gmail.com>

Signed-off-by: Ayush Singh <ayushdevel1325@gmail.com>

- Fixed some problems found during testing
- Mikrobus driver is not present in 6.6 kernel yet, so need to wait or
  add instructions to compile driver out of tree.

Signed-off-by: Ayush Singh <ayushdevel1325@gmail.com>

- A very rough first draft.
- Not completely tested.

Signed-off-by: Ayush Singh <ayushdevel1325@gmail.com>

Add OSHW mark for all boards

See merge request docs/docs.beagleboard.io!60