[ARM] 5308/1: Fix Viper ISA IRQ handling

The ISA IRQ renumbering broke the Viper ISA code in interesting ways.
It originally assumed that ISA interrupt were numbered in the order that
is defined by the CPLD registers. Unfortunately, this is no longer the
case.

Furthermore, the viper_irq_handler() function being a chained IRQ
handler, it must ACK the interrupt by itself, or the handler will be
immediately reentered, with the expected damages.

This fix was made possible thanks to the help of David Raeman, who
provided debug information and tested each version of this patch.

Tested-by: David Raeman <david.raeman@gmail.com>

Signed-off-by: Marc Zyngier <maz@misterjones.org>
Acked-by: Eric Miao <eric.miao@marvell.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>