arm: irq: Allow for specification of no preallocated irqs

For cases with SPARSE_IRQ enabled, irqs preallocated with
arch_probe_nr_irqs() are already marked as allocated in the
allocated_irqs bitmap. As a consequence, irq chip drivers that
allocate irqs will feel one of two behaviors:

1. An allocation will succeed with the starting irq_base one
more than the preallocated irqs. This will thus waste the
preceeding interrupt resources that were preallocated, unless a
legacy chip driver happens to assume ownership of these by some
platform definition. The GIC driver is a typical primary chip
driver, and abides to the allocation APIs. So this can be a
problem in many trivial usecases.

2. An allocation will fail with < 0. This can also happen in the
GIC driver, which interprets this value as meaning the irq_descs
are already preallocated. But in Device Tree configurations, the
fallback irq_base is -1. This results in an invalid irq_base
value.

Looking forward, we are moving towards a world where preallocation
of irqs is no longer necessary. irq_domain is scoped to handle all
irq_desc allocations in the future. Thus, we should support
configurations where the platform wants to preallocate no irqs.

One easy way to achieve this is to allow for
machine_desc->nr_irqs < 0, which indicates not to preallocate any
interrupts.

Change-Id: Ie793932c58de72c1b91b6e039b77a8e5d64ecc75
Signed-off-by: Michael Bohan <mbohan@codeaurora.org>
(cherry picked from commit 0bb2b56f7048b2f85be6784eaa8e4a5f0fa8688d)