[POWERPC] Fix storcenter DTS typos, feedback, IRQs.

Cleaned up IRQ layout and removed unsused ISU allocations.
Fixed RTC address typo from /dts-v1/ conversion.
Incorporated list suggestions to use an "iomega," vendor prefix,
and to use a node reference rather than a hard path.

Signed-off-by: Jon Loeliger <jdl@@jdl.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>