iommu/omap: Use WARN_ON for page table alignment check

The OMAP IOMMU page table needs to be aligned on a 16K boundary,
and the current code uses a BUG_ON on the alignment sanity check
in the .domain_alloc() ops implementation. Replace this with a
less severe WARN_ON and bail out gracefully.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>