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/mm/fault.c b/arch/tile/mm/fault.c
index 94f579d..f295b4a 100644
--- a/arch/tile/mm/fault.c
+++ b/arch/tile/mm/fault.c
@@ -563,10 +563,10 @@
 /*
  * When we take an ITLB or DTLB fault or access violation in the
  * supervisor while the critical section bit is set, the hypervisor is
- * reluctant to write new values into the EX_CONTEXT_1_x registers,
+ * reluctant to write new values into the EX_CONTEXT_K_x registers,
  * since that might indicate we have not yet squirreled the SPR
  * contents away and can thus safely take a recursive interrupt.
- * Accordingly, the hypervisor passes us the PC via SYSTEM_SAVE_1_2.
+ * Accordingly, the hypervisor passes us the PC via SYSTEM_SAVE_K_2.
  *
  * Note that this routine is called before homecache_tlb_defer_enter(),
  * which means that we can properly unlock any atomics that might
@@ -610,7 +610,7 @@
 	 * fault.  We didn't set up a kernel stack on initial entry to
 	 * sys_cmpxchg, but instead had one set up by the fault, which
 	 * (because sys_cmpxchg never releases ICS) came to us via the
-	 * SYSTEM_SAVE_1_2 mechanism, and thus EX_CONTEXT_1_[01] are
+	 * SYSTEM_SAVE_K_2 mechanism, and thus EX_CONTEXT_K_[01] are
 	 * still referencing the original user code.  We release the
 	 * atomic lock and rewrite pt_regs so that it appears that we
 	 * came from user-space directly, and after we finish the