sparc: Restore irq_trans_init() call in prom_create_node().

This broke sparc64 in various ways.

Add an empty dummy hook in sparc32's prom_32.c so that we
can potentially handle things on that side similarly, and
in particular avoid a prom_common.c ifdef :-)

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