use DMA_BIT_MASK instead of inline constant

Use DMA_BIT_MASK(24) instead of 0x00ffffff in DMA-mapping.txt

Signed-off-by: Marin Mitov <mitov@issp.bas.bg>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>