targets,runtime,machine: add support for the stm32f469-disco board

The LEDs and button work; I haven't tested the SPI and I2C
configuration.

src/machine/machine_stm32f469.go

@@ -0,0 +1,14 @@
+//go:build stm32f4 && stm32f469
+// +build stm32f4,stm32f469
+
+package machine
+
+func CPUFrequency() uint32 {
+	return 180000000
+}
+
+// Internal use: configured speed of the APB1 and APB2 timers, this should be kept
+// in sync with any changes to runtime package which configures the oscillators
+// and clock frequencies
+const APB1_TIM_FREQ = 45000000 * 2
+const APB2_TIM_FREQ = 90000000 * 2