External Interrupts

Anyone knows how to use external interrupts on mini2440?

Where do you want to use this feature? In your own kernel driver or in your
application?
In your kernel driver: take a look into other S3C2440 specific drivers
which uses GPIOs as their interrupt sources (refer kernel's
Documentation/gpio.txt for more details)
In your application: export an GPIO, define the "edge" you are interested
in and do a select() on the GPIO's "value" entry to wait for an interrupt
(also refer kernel's Documentation/gpio.txt here for more details)