Add RISC-V 32 IMAC support.

Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>

mcu/chx-gd32vf103.c

@@ -0,0 +1,22 @@
+#include <stdint.h>
+#include <mcu/gd32vf103.h>
+
+asm (
+	".equ	wfe,0x810\n\t"    /* Not used (yet). */
+	".equ	sleepvalue,0x811" /* Not used (yet). */
+);
+
+extern int chx_allow_sleep;
+
+void
+chx_sleep_mode (int how)
+{
+  /*TBD*/
+  (void)how;
+}
+
+void
+chx_prepare_sleep_mode (void)
+{
+  /*TBD*/
+}