sound/oss: remove VIRT_TO_BUS dependency

The OSS sound drivers used to rely on virt_to_bus(), but don't any more,
so we can remove the Kconfig dependency.

As a lot of architectures don't provide VIRT_TO_BUS any more, removing
the dependency in sounds/oss/ would make the deprecated drivers appear
there, which we probably don't want. Instead I'm replacing the
simple dependency with 'VIRT_TO_BUS || RPC || NETWINDER' so we can
still build these sound drivers for the platforms that need them,
but don't change anything on other architectures.

As a follow-up, we can remove the virt_to_bus() implementation
and Kconfig symbol in the ARM architecture.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>