ARM: OMAP2: twl-common: Fix compiler warning

If the kernel is built only for OMAP2 the following warning will show up:

arch/arm/mach-omap2/twl-common.c:52: warning: ‘twl_set_voltage’ defined but not used
arch/arm/mach-omap2/twl-common.c:58: warning: ‘twl_get_voltage’ defined but not used

The twl_set/get_voltage callbacks only used when OMAP3/4 is selected.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>