powerpc: Increase NR_IRQS Kconfig maximum to 32768

With dynamic irq descriptors the overhead of a large NR_IRQS is much lower
than it used to be. With more MSI-X capable adapters and drivers exploiting
multiple vectors we may as well allow the user to increase it beyond the
current maximum of 512.

32768 seems large enough that we'd never have to bump it again (although I bet
my prediction is horribly wrong). It boot tests OK and the vmlinux footprint
increase is only around 500kB due to:

struct irq_map_entry irq_map[NR_IRQS];

We format /proc/interrupts correctly with the previous changes:

             CPU0       CPU1       CPU2       CPU3       CPU4       CPU5
  286:          0          0          0          0          0          0
  516:          0          0          0          0          0          0
16689:       1833          0          0          0          0          0
17157:          0          0          0          0          0          0
17158:        319          0          0          0          0          0
25092:          0          0          0          0          0          0

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>