ARM: OMAP4: voltage: add support for VOLTSETUP_x_OFF register

OMAP4 has two VOLTSETUP registers. One is controlling retention and
sleep voltage setup times, the other one off mode setup times. Both
of these need to be setup for stable behavior of the device.
The code setting up the new register will be added in the next
patch.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>