input: Rewrite sparcspkr device probing.

Remove all dependencies on EBUS and ISA bus layers, which we'd like to
remove as they are superfluous.

While we're here, add support for proper frequency changing on BBC
beep devices.  Unlike the comments that were here, this device can
in fact use a programmable frequency.

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