sparc32,leon: operate on boot-cpu IRQ controller registers

* proper initialization of boot_cpu_id (no hardcoding to 0)
 * use boot_cpu_id index to address into the IRQ controller where
   appropriate

Each CPU has a separate set of IRQ controller registers, this
patch makes sure that the boot-cpu registers are used instead
of CPU0's.

Signed-off-by: Daniel Hellstrom <daniel@gaisler.com>
Signed-off-by: David S. Miller <davem@davemloft.net>