sparc: Kill SBUS DVMA layer.

This thing was completely pointless.

Just find the OF device in the parent of drivers that want to program
this device, and map the DMA regs inside such drivers too.

This also moves the dummy claim_dma_lock() and release_dma_lock()
implementation to floppy_32.h, which makes it handle this issue
just like floppy_64.h does.

Signed-off-by: David S. Miller <davem@davemloft.net>