arm64: mm: dma: Use dma attribute to allow executeable mappings

The default mapping type is non-executeable. Check for the
DMA_ATTR_EXEC_MAPPING attribute which allows clients to
request an executeable mapping.

Change-Id: I24a170990cc04a848b6779871ae2025721177d46
Signed-off-by: Rohit Vaswani <rvaswani@codeaurora.org>