ARM: OMAP2+: Fix realtime_counter_init warning in timer.c

ARM: OMAP2+: Fix realtime_counter_init warning in timer.c

In commit fa6d79d (ARM: OMAP: Add initialisation for the real-time
counter), the function realtime_counter_init() was added. However, if
the kernel configuration option CONFIG_SOC_OMAP5 is not selected then
the following compiler warning is observed.

  CC      arch/arm/mach-omap2/timer.o
  arch/arm/mach-omap2/timer.c:489:20: warning: ‘realtime_counter_init’
  defined but not used [-Wunused-function]

Commit fa6d79d also introduced the kernel configuration option
CONFIG_SOC_HAS_REALTIME_COUNTER. If this option is not selected then the
a stub function for realtime_counter_init() is defined.

For non-OMAP5 devices, there is no realtime counter and so
realtime_counter_init() function and stub function are not used for
these devices. Therefore, fix this warning by only allowing the kernel
configuration option CONFIG_SOC_HAS_REALTIME_COUNTER to be enabled for
OMAP5 devices.

Cc: Santosh Shilimkar <santosh.shilimkar@ti.com>
Reported-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Jon Hunter <jon-hunter@ti.com>
Acked-by Santosh Shilimkar <santosh.shilimkar@ti.com>