ARM: integrator: get rid of preallocated irq descriptors

The Integrators were using the .nr_irqs field of the machine
descriptor to pre-allocate a number of descriptors at boot. This
is not right: the irq chip implementations should allocate
their descriptors themselves, and as a result the simple
irqdomain code warns about it.

Get rid of this by just deleting the .nr_irq field from the
machine descriptors but take care: doing so makes the default
implementation hog the first 16 IRQ numbers, so these cannot
be used by the still static IRQ number assignments in the
ATAG boot case. So for these, bump the IRQ numbers to begin
at 64 and upward.

Introduce an offset to offset all IRQ numbers if need be,
though we don't expect to do that again as device tree comes
along.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>