ARM: pxa: move mach/sound.h to linux/platform_data/

This is a basically a platform_data file, so move it out of
the mach/* header directory.

Cc: Marek Vasut <marek.vasut@gmail.com>
Cc: Tomas Cech <sleep_walker@suse.com>
Cc: Sergey Lapin <slapin@ossfans.org>
Acked-by: Mark Brown <broonie@kernel.org>
Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>
Cc: alsa-devel@alsa-project.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>