i2csoft: add source code for each target

i2csoft/i2csoft_nrf52840.go

@@ -0,0 +1,17 @@
+//go:build nrf52840
+// +build nrf52840
+
+package i2csoft
+
+import (
+	"device"
+)
+
+// wait waits for half the time of the SCL operation interval. It is set to
+// about 100 kHz.
+func (i2c *I2C) wait() {
+	wait := 26
+	for i := 0; i < wait; i++ {
+		device.Asm(`nop`)
+	}
+}