asm-generic/dma-mapping-broken.h: Provide dma_alloc_attrs()/dma_free_attrs()

Since commit 0049fb26 ("OMAPFB: use
dma_alloc_attrs to allocate memory") we have one non-arch user of
dma_{alloc,free}_attrs().

Hence provide these functions, as wrappers around
dma_{alloc,free}_coherent().

Note that most architectures do it the other way around. But as these are
dummy functions, we don't care.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>