sparc32: Convert mmu_* interfaces from btfixup to method ops.

This set of changes displays one major danger of btfixup, interface
signatures are not always type checked fully.  As seen here the iounit
variant of the map_dma_area routine had an incorrect type for one of
it's arguments.

It turns out to be harmless in this case, but just imagine trying to
debug something involving this kind of problem.  No thanks.

Signed-off-by: David S. Miller <davem@davemloft.net>