As Sun 3 kernels cannot be multi-platform due to the different Sun 3 MMU
type, it made sense to statically allocate the table to track IOMMU use.

However, Sun 3x kernels can be multi-platform. Furthermore, Sun 3x uses
a larger table than Sun 3 (8192 bytes instead of 512 bytes).

Hence switch to dynamic allocation of this table using the bootmem
allocator to avoid wasting 8192 bytes when not running on a Sun 3x.
As this allocator returns zeroed memory, there's no need to explicitly
initialize the table to zeroes.
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>