PSCI: Update comment on MMU disablement

The MMU is not disabled in this path, update the comment to
reflect this. Also clarify that both paths call prepare_cpu_pwr_dwn(),
but the second path does stack cache maintenance.
Signed-off-by: Andrew F. Davis <afd@ti.com>

lib/psci/psci_common.c

@@ -948,21 +948,18 @@ void psci_do_pwrdown_sequence(unsigned int power_level)
 	/*
 	 * With hardware-assisted coherency, the CPU drivers only initiate the
 	 * power down sequence, without performing cache-maintenance operations
-	 * in software. Data caches and MMU remain enabled both before and after
-	 * this call.
+	 * in software. Data caches enabled both before and after this call.
 	 */
 	prepare_cpu_pwr_dwn(power_level);
 #else
 	/*
 	 * Without hardware-assisted coherency, the CPU drivers disable data
-	 * caches and MMU, then perform cache-maintenance operations in
-	 * software.
+	 * caches, then perform cache-maintenance operations in software.
 	 *
-	 * We ought to call prepare_cpu_pwr_dwn() to initiate power down
-	 * sequence. We currently have data caches and MMU enabled, but the
-	 * function will return with data caches and MMU disabled. We must
-	 * ensure that the stack memory is flushed out to memory before we start
-	 * popping from it again.
+	 * This also calls prepare_cpu_pwr_dwn() to initiate power down
+	 * sequence, but that function will return with data caches disabled.
+	 * We must ensure that the stack memory is flushed out to memory before
+	 * we start popping from it again.
 	 */
 	psci_do_pwrdown_cache_maintenance(power_level);
 #endif