sparc32,leon: add support for extended interrupt controller

The extended IRQ controller gives the LEON 16 more IRQs.

The patch installs a custom handler for the exetended controller
IRQ, where a register is read and the "real" IRQ causing IRQ is
determined.

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