common: dma-mapping: remove old alloc_coherent and free_coherent methods

Remove old, unused alloc_coherent and free_coherent methods from
dma_map_ops structure.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>