openrisc: fix initial preempt state for secondary cpu tasks

During SMP testing we were getting the below warning after booting the
secondary cpu:

[    0.060000] BUG: scheduling while atomic: swapper/1/0/0x00000000

This change follows similar patterns from other architectures to start
the schduler with preempt disabled.

Signed-off-by: Stafford Horne <shorne@gmail.com>