powerpc/fsl-corenet: Disable coreint if kexec is enabled

Problems have been observed in coreint (EPR) mode if interrupts are
left pending (due to the lack of device quiescence with kdump) after
having tried to deliver to a CPU but unable to deliver due to MSR[EE]
-- interrupts no longer get reliably delivered in the new kernel.  I
tried various ways of fixing it up inside the crash kernel itself, and
none worked (including resetting the entire mpic).  Masking all
interrupts and issuing EOIs in the crashing kernel did help a lot of
the time, but the behavior was not consistent.

Thus, stick to standard IACK mode when kdump is a possibility.

Signed-off-by: Scott Wood <scottwood@freescale.com>