powerpc/cell: Only iterate over online nodes in cbe_init_pm_irq()

None of the cell platforms support CPU hotplug, so we should iterate
only over online nodes when setting PMU interrupts.

This also fixes a warning during boot when NODES_SHIFT is large enough:

WARNING: at /scratch/michael/src/kmk/linus/kernel/irq/irqdomain.c:766
...
NIP [c0000000000db278] .irq_linear_revmap+0x30/0x58
LR [c0000000000dc2a0] .irq_create_mapping+0x38/0x1a8
Call Trace:
[c0000003fc9c3af0] [c0000000000dc2a0] .irq_create_mapping+0x38/0x1a8 (unreliable)
[c0000003fc9c3b80] [c000000000655c1c] .__machine_initcall_cell_cbe_init_pm_irq+0x84/0x158
[c0000003fc9c3c20] [c00000000000afb4] .do_one_initcall+0x5c/0x1e0
[c0000003fc9c3cd0] [c000000000644580] .kernel_init_freeable+0x238/0x328
[c0000003fc9c3db0] [c00000000000b784] .kernel_init+0x1c/0x120
[c0000003fc9c3e30] [c000000000009fb8] .ret_from_kernel_thread+0x64/0xac

This is caused by us overflowing our linear revmap because we're
requesting too many interrupts.

Reported-by: Dennis Schridde <devurandom@gmx.net>
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>