common: DMA-mapping: Add EXEC_MAPPING attribute

DMA_ATTR_EXEC_MAPPING specifies that an executable mapping
should be created for the requested buffer. By default, the
DMA mappings are non-executable.

Change-Id: I135077e14996e92fa9d199bdee043c443db48924
Signed-off-by: Rohit Vaswani <rvaswani@codeaurora.org>