ARM: pxa: fix broken isa interrupts for zeus and viper

Commit "ARM: pxa: arbitrarily set first interrupt number" broke viper
and zeus boards which still refer to PXA_ISA_IRQ() macro. Redefine
the macro, which declares the legacy interrupts from 0 to 15.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Olof Johansson <olof@lixom.net>