omap2plus: voltage: Trivial linking fix 'undefined reference'

LD      init/built-in.o
LD      .tmp_vmlinux1
arch/arm/mach-omap2/built-in.o: In function `omap2_set_init_voltage':
arch/arm/mach-omap2/pm.c:181: undefined reference to `omap_voltage_domain_lookup'
arch/arm/mach-omap2/built-in.o: In function `omap4_twl_init':
arch/arm/mach-omap2/omap_twl.c:244: undefined reference to `omap_voltage_domain_lookup'
arch/arm/mach-omap2/omap_twl.c:247: undefined reference to `omap_voltage_domain_lookup'
arch/arm/mach-omap2/omap_twl.c:250: undefined reference to `omap_voltage_domain_lookup'
make: *** [.tmp_vmlinux1] Error 1

The error is reported when omap2plus_defconfig built with CONFIG_PM disabled

Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Cc: Thara Gopinath <thara@ti.com>
Cc: Kevin Hilman <khilman@deeprootsystems.com>
Cc: Nishanth Menon <nm@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>