feat(ch32f20x): add support of ch32f20x

hw/bsp/ch32f20x/system_ch32f20x.h

@@ -0,0 +1,28 @@
+/********************************** (C) COPYRIGHT *******************************
+* File Name          : system_ch32f20x.h
+* Author             : WCH
+* Version            : V1.0.0
+* Date               : 2021/08/08
+* Description        : CH32F20x Device Peripheral Access Layer System Header File.
+*******************************************************************************/
+#ifndef __SYSTEM_CH32F20x_H
+#define __SYSTEM_CH32F20x_H
+
+#ifdef __cplusplus
+ extern "C" {
+#endif
+
+extern uint32_t SystemCoreClock;          /* System Clock Frequency (Core Clock) */
+
+/* System_Exported_Functions */
+extern void SystemInit(void);
+extern void SystemCoreClockUpdate(void);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /*__CH32F20x_SYSTEM_H */
+
+
+