[POWERPC] cpm2: CPM2 interrupt controller fix

This contains important fixes for the CPM2 PIC code. Eliminated
CPM_IRQ_OFFSET, pulling the respective interrupt numbers from the interrupt
mapping. Updated devicetree files to reflect that. Changed direct
IC-related IO accesses to the IO accessors. Fixed all the sense values to
keep coherency with ipic. In the current code, CPM2 stuff will have no IRQs
and hence could be hardly usable.

Signed-off-by: Vitaly Bordug <vbordug@ru.mvista.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>