ARM: OMAP: add RCU locking to omap2_set_init_voltage.

opp_find_freq_ceil and opp_get_voltage are documented as requiring
rcu_lock to be held.  So hold it.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Kevin Hilman <khilman@ti.com>