powerpc/powernv: Move opal_power_control_init() call in opal_init().

opal_power_control_init() depends on opal message notifier to be
initialized, which is done in opal_init()->opal_message_init(). But both
these initialization are called through machine initcalls and it all
depends on in which order they being called. So far these are called in
correct order (may be we got lucky) and never saw any issue. But it is
clearer to control initialization order explicitly by moving
opal_power_control_init() into opal_init().

Signed-off-by: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>