ARM: pxa: arbitrarily set first interrupt number

As IRQ0, the legacy timer interrupt  should not be used as an interrupt
number, shift the interrupts by a fixed number.

As we had in a special case a shift of 16 when ISA bus was used on a
PXA, use that value as the first interrupt number, regardless of ISA or
not.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>