ARM: OMAP: make ti81xx_rtc_hwmod static

The ti81xx_rtc_hwmod is not exported, or declared outside the file
arch/arm/mach-omap2/omap_hwmod_81xx_data.c so make it static to
avoid the following warning:

arch/arm/mach-omap2/omap_hwmod_81xx_data.c:246:19: warning: symbol 'ti81xx_rtc_hwmod' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Tony Lindgren <tony@atomide.com>