ARM: OMAP3: control: add API for setting IVA bootmode

OMAP3 PM core requires IVA2 bootmode to be set to idle during init. Currently,
a direct register write is used for this. Add a new ctrl API for this purpose
instead.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>