microblaze: fix divide by zero exception message

Fix divide exception message to say "divide by zero".

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc:	Michal Simek <monstr@monstr.eu>
Cc:	microblaze-uclinux@itee.uq.edu.au
Signed-off-by: Michal Simek <monstr@monstr.eu>