add stm32 nucleol476rg support

src/runtime/runtime_stm32l4x6.go

@@ -0,0 +1,29 @@
+//go:build stm32 && stm32l4x6
+
+package runtime
+
+import (
+	"device/stm32"
+)
+
+/*
+clock settings
+
+	+-------------+-----------+
+	| LSE         | 32.768khz |
+	| SYSCLK      | 80mhz    |
+	| HCLK        | 80mhz    |
+	| APB1(PCLK1) | 80mhz    |
+	| APB2(PCLK2) | 80mhz    |
+	+-------------+-----------+
+*/
+const (
+	HSE_STARTUP_TIMEOUT = 0x0500
+	PLL_M               = 1
+	PLL_N               = 40
+	PLL_P               = RCC_PLLP_DIV7
+	PLL_Q               = RCC_PLLQ_DIV2
+	PLL_R               = RCC_PLLR_DIV2
+
+	MSIRANGE = stm32.RCC_CR_MSIRANGE_Range4M
+)