bmi270: add driver for BMI270 6-axis IMU

[t-morisawa]

Summary

Add I2C driver for the Bosch BMI270 6-axis inertial measurement unit.

Features

• Configurable accelerometer range (±2g/4g/8g/16g)
• Configurable gyroscope range (±125/250/500/1000/2000 dps)
• ReadAcceleration returns values in µg
• ReadRotation returns values in µdps
• Uses go:embed for firmware config data (same pattern as bma42x)
• Follows existing driver conventions (reg_ prefix, uint8 address, etc.)

Files

• bmi270/bmi270.go - driver implementation
• bmi270/registers.go - register definitions
• bmi270/bmi270-config.bin - BMI270 firmware config data
• examples/bmi270/main.go - example usage

Testing

test M5Stack Core 1.3

acc(mg): 8 -506 799  gyr(dps): -277 39 -25
acc(mg): -119 -864 353  gyr(dps): -231 68 151
acc(mg): -271 -925 304  gyr(dps): -45 42 105
acc(mg): -424 -830 498  gyr(dps): 160 18 113
acc(mg): -363 -723 883  gyr(dps): 198 -75 -28

the test program is below:

func main() {
	i2c := machine.I2C0
	i2c.Configure(machine.I2CConfig{
		Frequency: 400e3,
		SDA:       machine.SDA0_PIN,
		SCL:       machine.SCL0_PIN,
	})

	imu := bmi270.NewI2C(i2c, bmi270.Address)
	if err := imu.Configure(bmi270.DefaultConfig()); err != nil {
		println("BMI270 error:", err.Error())
		return
	}
	println("BMI270 initialized")

	for {
		ax, ay, az, err := imu.ReadAcceleration()
		if err != nil {
			println("accel error:", err.Error())
			time.Sleep(100 * time.Millisecond)
			continue
		}
		gx, gy, gz, err := imu.ReadRotation()
		if err != nil {
			println("gyro error:", err.Error())
			time.Sleep(100 * time.Millisecond)
			continue
		}

		print("acc(mg): ", ax/1000, " ", ay/1000, " ", az/1000)
		print("  gyr(dps): ", gx/1000000, " ", gy/1000000, " ", gz/1000000)
		println()

		time.Sleep(100 * time.Millisecond)
	}
}

Closes #872