stm32: add minimal stm32wlex5 / lorae5 target

credit to ofauchon

src/machine/machine_stm32_exti_syscfg_noenable.go

@@ -0,0 +1,27 @@
+//go:build stm32wle5
+// +build stm32wle5
+
+package machine
+
+import (
+	"device/stm32"
+	"runtime/volatile"
+)
+
+func getEXTIConfigRegister(pin uint8) *volatile.Register32 {
+	switch (pin & 0xf) / 4 {
+	case 0:
+		return &stm32.SYSCFG.EXTICR1
+	case 1:
+		return &stm32.SYSCFG.EXTICR2
+	case 2:
+		return &stm32.SYSCFG.EXTICR3
+	case 3:
+		return &stm32.SYSCFG.EXTICR4
+	}
+	return nil
+}
+
+func enableEXTIConfigRegisters() {
+	// No registers to enable
+}