staging: xillybus: Use devm_ API for memory allocation and DMA mapping

Managed device resource API replaces code that reinvents it for memory
allocation, page allocation and DMA mapping.

devm_add_action() is used for unwinding DMA mappings, since there is no
devm_* API for dma_map_single(). A recent patch that introduces such API
was rejected, mainly on the grounds that it may cause an unnecessary waste of
resources.

Suggested-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Eli Billauer <eli.billauer@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>