sparc32: mm: Reduce allocation size for PMD and PTE tables

Now that the page table allocator can free page table allocations
smaller than PAGE_SIZE, reduce the size of the PMD and PTE allocations
to avoid needlessly wasting memory.

Cc: "David S. Miller" <davem@davemloft.net>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>