sparc32, leon: Enable interrupts before going idle to avoid getting stuck

This enables interrupts for Leon before having the CPU enter power-down mode.

Commit 87fa05ae, "sparc: Use generic idle loop",
gets the CPU stuck on idle for Leon systems. On Leon, disabling interrupts and
powering down the processor will get the processor stuck waiting for an
interrupt that will never be reacted to.

Signed-off-by: Andreas Larsson <andreas@gaisler.com>
Signed-off-by: David S. Miller <davem@davemloft.net>