[sw/dif_pwrmgr] Power Manager DIF Library Header

This change introduces the header of the power manager DIF library that
defines its public interface.

Signed-off-by: Alphan Ulusoy <alphan@google.com>

sw/device/lib/dif/dif_pwrmgr.c

diff --git a/sw/device/lib/dif/dif_pwrmgr.c b/sw/device/lib/dif/dif_pwrmgr.c
new file mode 100644
index 0000000..e82c31d
--- /dev/null
+++ b/sw/device/lib/dif/dif_pwrmgr.c
@@ -0,0 +1,11 @@
+// Copyright lowRISC contributors.
+// Licensed under the Apache License, Version 2.0, see LICENSE for details.
+// SPDX-License-Identifier: Apache-2.0
+
+#include "sw/device/lib/dif/dif_pwrmgr.h"
+
+#include "sw/device/lib/common.h"
+
+#include "pwrmgr_regs.h"  // Generated.
+
+#error "Power Manager DIF library is not implemented yet."