powerpc/pseries: Fix getting the server number size

The 'ibm,interrupt-server#-size' properties are not in the cpu nodes,
which is where we currently look for them, but rather live under the
interrupt source controller nodes (which have "ibm,ppc-xics" in their
compatible property).

This moves the code that looks for the ibm,interrupt-server#-size
properties from xics_update_irq_servers() into xics_init_IRQ().

Also this adds a check for mismatched sizes across the interrupt
source controller nodes.  Not sure this is necessary as in this case
the firmware might be seriously busted.

This property only appears on POWER6 boxes and is only used in the
set-indicator(gqirm) call, and apparently firmware currently ignores
the value we pass.  Nevertheless we need to fix it in case future
firmware versions use it.

Signed-off-by: Sebastien Dugue <sebastien.dugue@bull.net>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>