atl1: explain 32-bit DMA restriction

Document the fact that atl1 uses a single shared register for the high 32
bits of 64-bit DMA addresses, making 64-bit DMA more trouble than it's worth.

Signed-off-by: Chris Snook <csnook@redhat.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>