lib: add poweroff() declaration

Add a function to power off the system within the halt.h header.

BUG=chrome-os-partner:54977

Change-Id: I21ca9de38d4ca67c77272031cc20f3f1d015f8fa
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/15684
Reviewed-by: Furquan Shaikh <furquan@google.com>
Tested-by: build bot (Jenkins)

src/include/halt.h

@@ -25,4 +25,8 @@
  */
 void __attribute__((noreturn)) halt(void);
 #endif /* __ROMCC__ */
+
+/* Power off the system. */
+void poweroff(void);
+
 #endif /* __HALT_H__ */