nios2: Add a reset_cpu() function

The current do_reset() is called from a command context. Add a function
which can be used from anywhere, as is done on ARM.

Signed-off-by: Simon Glass <sjg@chromium.org>

arch/nios2/cpu/cpu.c

@@ -35,11 +35,17 @@ int checkboard(void)
 }
 #endif
 
-int do_reset(struct cmd_tbl *cmdtp, int flag, int argc, char *const argv[])
+void reset_cpu(void)
 {
 	disable_interrupts();
 	/* indirect call to go beyond 256MB limitation of toolchain */
 	nios2_callr(gd->arch.reset_addr);
+}
+
+int do_reset(struct cmd_tbl *cmdtp, int flag, int argc, char *const argv[])
+{
+	reset_cpu();
+
 	return 0;
 }