Currently we're attempting to use a static value for the
voltctrl register that only works for controlling the PMIC
over I2C4. For using sys_off_mode signaling, we need to update
update clksetup, voltsetup1, voltsetup2 and voltctrl registers
dynamically depending on the idle state.

So let's fix this by configuring things for I2C4 controlled idle
and sys_off_mode pin controlled idle, and then write the
configured register values depending on the idle state. This
is similar what N900 kernel is doing too.

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