add stm32 nucleol476rg support

src/machine/machine_stm32l4x6.go

@@ -0,0 +1,25 @@
+//go:build stm32l4x6
+
+package machine
+
+// Peripheral abstraction layer for the stm32l4x6
+
+func CPUFrequency() uint32 {
+	return 80e6
+}
+
+// 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 = 80e6 // 80MHz
+const APB2_TIM_FREQ = 80e6 // 80MHz
+
+//---------- I2C related code
+
+// Gets the value for TIMINGR register
+func (i2c *I2C) getFreqRange() uint32 {
+	// This is a 'magic' value calculated by STM32CubeMX
+	// for 80MHz PCLK1.
+	// TODO: Do calculations based on PCLK1
+	return 0x10909CEC
+}