hd64461 is mapped in a fixed location, so the I/O base itself is fairly
meaningless as a configuration item. Additionally, this makes it
impossible to share hd64461 code alongside generic drivers (in the case
of sh_dac_audio), so simply make it commonly defined and permit the
mach_is_foo() logic to work out the proper semantics.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>