arch:arm:plat-omap:iovmm: remove unused variable 'va'

The pointer "va" returned from "phys_to_virt(pa)" is never used in
"sgtable_fill_kmalloc()".So,it is safe to remove this set-but-unused variable.

Signed-off-by: Maxin B. John <maxin.john@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>