Introduce new generic mmap method with attributes argument.

This method lets drivers to create a userspace mapping for a DMA buffer
in generic, architecture independent way.
Signed-off-by: NMarek Szyprowski <m.szyprowski@samsung.com>
Acked-by: NKyungmin Park <kyungmin.park@samsung.com>
Reviewed-by: NArnd Bergmann <arnd@arndb.de>