ARM: imx/pcm970: make platform_data static

This fixes the following sparse warning:

	arch/arm/mach-imx/pcm970-baseboard.c:224:13: warning: symbol 'pcm970_baseboard_init' was not declared. Should it be static?

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>