Add ability to debug clock with MCO1

src/gpio.h

@@ -18,6 +18,7 @@ struct gpio {
 };
 
 // AFRH, AFRL registers
+#define GPIO_AF_MCO_1 (0b0000)     // Alternative function 0 (AF0)
 #define GPIO_AF_USART2_RX (0b0111) // Alternative function 7 (AF7)
 #define GPIO_AF_USART2_TX (0b0111) // Alternative function 7 (AF7)