sparc32: mm: Change pgtable_t type to pte_t * instead of struct page *

Change the 'pgtable_t' type for sparc32 so that it represents the uncached
virtual address of the PTE table, rather than the underlying 'struct page'.

This allows us to free page table allocations smaller than a page.

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>