ARM: OMAP2+: Fix sparse warnings in timer.c

ARM: OMAP2+: Fix sparse warnings in timer.c

Sparse generates the following warnings when compiling mach-omap2/timer.c.

  CHECK   arch/arm/mach-omap2/timer.c
  arch/arm/mach-omap2/timer.c:193:13: warning: symbol 'omap_dmtimer_init'
  was not declared. Should it be static?
  arch/arm/mach-omap2/timer.c:213:12: warning: symbol
  'omap_dm_timer_get_errata' was not declared. Should it be static?

Add static to function declaration to fix warnings.

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