swiotlb: Setting default IO TBL value to 1MB

swiotlb was allocting the 64MB of memory at boot up time, because we
used CMA so we do not use it. We can reduce it to 1MB.

Change-Id: I137f6e9069f86dcc9dcd883868b3e8a08c29d710
Signed-off-by: Abhimanyu Garg <agarg@codeaurora.org>