ARM: OMAP: Fix warning about missing clk_get_usecount() declaration

arch/arm/plat-omap/clock.c exports clk_get_usecount(), but linux/clk.h
doesn't contain the prototype for it. Add it to asm/arch/clock.h and add
clock.h to omap-audio-aic23.c.

sound/oss/omap-audio-aic23.c: In function `audio_aic23_init':
sound/oss/omap-audio-aic23.c:680: warning: implicit declaration of
function `clk_get_usecount'

Signed-off-by: Dirk Behme <dirk.behme_at_de.bosch.com>