[CORE] Introduce dma-coherent memory flag.

Moving forward, we will soon encounter DMA-capable devices.
The drivers for such DMA-capable devices may require DMA
coherent memory for using built-in DMA.

The patch introduces VMM_MEMORY_DMACOHERENT memory flag
which can be used to request dma-coherent pages.

Signed-off-by: Anup Patel <anup@brainfault.org>