stm32f7:DMA add dcache alignment check in stm32_dmacapable

   In the case dcache write-buffed mode is used (not write-through)
   buffer alignment is required for DMA transfers because
   a) arch_invalidate_dcache could lose buffered writes data
   and b) arch_flush_dcache could corrupt adjacent memory if
   the maddr and the mend+1, the next next address are not on
   ARMV7M_DCACHE_LINESIZE boundaries.