ARM: OMAP2+: Fix undefined reference to set_cntfreq

If CONFIG_SOC_HAS_REALTIME_COUNTER is not selected for omap5
or dra7xx, we can get the following error:

arch/arm/mach-omap2/built-in.o: In function `omap4_secondary_init':
:(.text+0x7ab0): undefined reference to `set_cntfreq'

Fix the issue by not trying to initalize the realtime counter
unles CONFIG_SOC_HAS_REALTIME_COUNTER is selected.

Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>