Prevent crash on division by zero

Currently, attempting to divide by zero in an integer expression in a dts
file will cause dtc to crash with a division by zero (SIGFPE).

This patch corrects this to properly detect this case and raise an error.

Reported-by: Anton Blanchard <anton@samba.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>