[ARM] Fix Integrator and Versatile interrupt initialisation

Both Integrator and Versatile were using set_irq_handler() and
enable_irq(), and working around the initialisation of the
chained interrupt, instead of the more correct
set_irq_chained_handler() function.  Fix Integrator and
Versatile to use the right function, and remove these work-arounds.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>