arm: dma-mapping: handle IOVA address zero

The IOVA allocator used by these calls supports IOVA address
zero so properly handle IOVA address zero.

Change-Id: Iaaad45b42503f05f95d1333e010f6d8e3bdb1949
Signed-off-by: Liam Mark <lmark@codeaurora.org>
Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>
Signed-off-by: Vijayanand Jitta <vjitta@codeaurora.org>
Signed-off-by: Qingqing Zhou <qqzhou@codeaurora.org>