ARM: OMAP3: PM: remove access to PRM_VOLTCTRL register

There is a solitary write to this register every wakeup from off-mode,
which isn't doing anything, so remove it.

Also note that modifying this register trashes any attempted
voltage scaling configuration and the change probably should
never have gotten merged in the first place.

Cc: Nishanth Menon <nm@ti.com>
Cc: Kevin Hilman <khilman@linaro.org>
Cc: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tero Kristo <t-kristo@ti.com>
[tony@atomide.com: updated comments to describe regression]
Signed-off-by: Tony Lindgren <tony@atomide.com>