irq_domain/powerpc: eliminate irq_map; use irq_alloc_desc() instead

This patch drops the powerpc-specific irq_map table and replaces it with
directly using the irq_alloc_desc()/irq_free_desc() interfaces for allocating
and freeing irq_desc structures.

This patch is a preparation step for generalizing the powerpc-specific virq
infrastructure to become irq_domains.

As part of this change, the irq_big_lock is changed to a mutex from a raw
spinlock.  There is no longer any need to use a spin lock since the irq_desc
allocation code is now responsible for the critical section of finding
an unused range of irq numbers.

The radix lookup table is also changed to store the irq_data pointer instead
of the irq_map entry since the irq_map is removed.  This should end up being
functionally equivalent since only allocated irq_descs are ever added to the
radix tree.

v5: - Really don't ever allocate virq 0.  The previous version could still
      do it if hint == 0
    - Respect irq_virq_count setting for NOMAP.  Some NOMAP domains cannot
      use virq values above irq_virq_count.
    - Use numa_node_id() when allocating irq_descs.  Ideally the API should
      obtain that value from the caller, but that touches a lot of call sites
      so will be deferred to a follow-on patch.
    - Fix irq_find_mapping() to include irq numbers lower than
      NUM_ISA_INTERRUPTS.  With the switch to irq_alloc_desc*(), the lowest
      possible allocated irq is now returned by arch_probe_nr_irqs().
v4: - Fix incorrect access to irq_data structure in debugfs code
    - Don't ever allocate virq 0

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Milton Miller <miltonm@bga.com>
Tested-by: Olof Johansson <olof@lixom.net>