ARM: OMAP2+: Fix randconfig build warning for dm814_pllss_data

Fix warning for arch/arm/mach-omap2/prm_common.c:666:35: warning:
‘dm814_pllss_data’ defined but not used [-Wunused-variable]".

This can happen if CONFIG_SOC_TI81XX is not selected.

Signed-off-by: Tony Lindgren <tony@atomide.com>