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 clk_get_usecount'

Signed-off-by: Dirk Behme <dirk.behme_at_de.bosch.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>