ARM: AM335x: Fix warning in timer.c

ARM: AM335x: Fix warning in timer.c

When compiling the kernel with configuration options ...

 # CONFIG_ARCH_OMAP2 is not set
 # CONFIG_ARCH_OMAP3 is not set
 # CONFIG_ARCH_OMAP4 is not set
 # CONFIG_SOC_OMAP5 is not set
 CONFIG_SOC_AM33XX=y

 ... the following build warning is seen.

  CC      arch/arm/mach-omap2/timer.o
  arch/arm/mach-omap2/timer.c:395:19: warning: ‘omap2_sync32k_clocksource_init’
  	defined but not used [-Wunused-function]

This issue was introduced by commit 6f80b3b (ARM: OMAP2+: timer: remove
CONFIG_OMAP_32K_TIMER) where the omap2_sync32k_clocksource_init() is no
longer referenced by the timer initialisation function for the AM335x
device as it has no 32k-sync timer.

Fix this by adding the "__maybe_unused" compiler directive to the
omap2_sync32k_clocksource_init() function to indicate that this function
may be used for certain configurations.

Cc: Igor Grinberg <grinberg@compulab.co.il>
Signed-off-by: Jon Hunter <jon-hunter@ti.com>