[SPARC]: Make SBUS dma code similar to EBUS

From: Georg Chini <georg.chini@triaton-webhosting.com>

Introduce some sbus_dma routines similar to the
ebus_dma stuff to make the code look nearly the same
for both cases.

Thanks to Christopher for testing.

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