pseries: Use #define for XICS base irq number

pseries: Use #define for XICS base irq number

Currently the lowest "real" irq number for the XICS irq controller (as
opposed to numbers reserved for IPIs and other special purposes) is
hard coded as 16 in two places - in xics_system_init() and in spapr.c.

As well as being generally bad practice, we're going to need to change this
number soon to fit in with the in-kernel XICS implementation.  This patch
adds a #define for this number to avoid future breakage.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Ben Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>