ASoC: kirkwood: make kirkwood_soc_platform const

Make kirkwood_soc_platform const as it only passed to a const argument
of the function snd_soc_register_platform in the file referencing it.
Make the declaration const too.

Done using Coccinelle.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>