ARM: OMAP4+: Fix sparse warning in system timers

Commit 6bb27d73 (ARM: delete struct sys_timer) changed the function
created by the macro OMAP_SYS_32K_TIMER_INIT from static void to void.
For OMAP4+ devices this created the following sparse warning ...

arch/arm/mach-omap2/timer.c:585:1: warning: symbol
	'omap4_sync32k_timer_init' was not declared. Should it be static?

The function omap4_sync32k_timer_init() is not referenced outside of the
file timer.c and so make this function static.

Signed-off-by: Jon Hunter <jon-hunter@ti.com>