ARM: OMAP2+: voltage: fixup oscillator handling when CONFIG_PM=n

commit 908b75e8

 (ARM: OMAP: add support for oscillator setup) added a new
API for oscillator setup, but is broken when CONFIG_PM=n.

The new functions have dummy definitions when CONFIG_PM=n, but also have
full implementations available, which conflict.

To fix, wrap the PM implmentations in #ifdef CONFIG_PM.

Cc: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>