arch/tile: parameterize system PLs to support KVM port

While not a port to KVM (yet), this change modifies the kernel
to be able to build either at PL1 or at PL2 with a suitable
config switch.  Pushing up this change avoids handling branch
merge issues going forward with the KVM work.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>
diff --git a/arch/tile/kernel/messaging.c b/arch/tile/kernel/messaging.c
index 6d23ed2..997e393 100644
--- a/arch/tile/kernel/messaging.c
+++ b/arch/tile/kernel/messaging.c
@@ -34,7 +34,7 @@
 		panic("hv_register_message_state: error %d", rc);
 
 	/* Make sure downcall interrupts will be enabled. */
-	raw_local_irq_unmask(INT_INTCTRL_1);
+	raw_local_irq_unmask(INT_INTCTRL_K);
 }
 
 void hv_message_intr(struct pt_regs *regs, int intnum)