sparc32, leon: must protect IRQ controller register with spinlock

The LEON interrupt controller has one single mask register for all
IRQs per CPU, even though the genirq layer protects us from accessing
the same IRQ at the same time other IRQs share the same mask register
and may thus interfere. Some other IRQ controllers has a mask register
or similar per IRQ instead which makes spinlocks unncessary.

Signed-off-by: Daniel Hellstrom <daniel@gaisler.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>