Patch from Lennert Buytenhek

IXP2000 interrupt source zero is a software-generated interrupt source,
but it is not an SWI in the ARM sense of the word.  Rename the interrupt
source to reduce any confusion.
  

Signed-off-by: Lennert Buytenhek
Signed-off-by: Deepak Saxena
Signed-off-by: Russell King