powerpc: iSeries has only 256 IRQs

The iSeries Hypervisor only allows us to specify IRQ numbers up to 255 (it
has a u8 field to pass it in).  This patch allows platforms to specify a
maximum to the virtual IRQ numbers we will use and has iSeries set that
to 255.  If not set, the maximum is NR_IRQS - 1 (as before).

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
diff --git a/arch/powerpc/platforms/iseries/setup.c b/arch/powerpc/platforms/iseries/setup.c
index 6ce8a40..a6fd9be 100644
--- a/arch/powerpc/platforms/iseries/setup.c
+++ b/arch/powerpc/platforms/iseries/setup.c
@@ -54,6 +54,7 @@
 #include <asm/iseries/hv_lp_event.h>
 #include <asm/iseries/lpar_map.h>
 #include <asm/udbg.h>
+#include <asm/irq.h>
 
 #include "naca.h"
 #include "setup.h"
@@ -684,6 +685,12 @@
 	powerpc_firmware_features |= FW_FEATURE_ISERIES;
 	powerpc_firmware_features |= FW_FEATURE_LPAR;
 
+	/*
+	 * The Hypervisor only allows us up to 256 interrupt
+	 * sources (the irq number is passed in a u8).
+	 */
+	virt_irq_max = 255;
+
 	return 1;
 }