powerpc/44x/fsp2: Interrupt handling setup

* clear out any possible plb6 errors
* board interrupt handling setup within l2 reg set
* fsp2 parity error setup

All those points are needed for correct interrupt
handling on board level including error handling report.

Reviewed-by: Alistair Popple <alistair@popple.id.au>
Signed-off-by: Ivan Mikhaylov <ivan@de.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>