[Kevin Townsend]

Updates the proposed LSM303DLHC magnetometer device following review by
Philippe Mathieu-Daudé.

This has been tested with Zephyr 2.6.0, as follows:

$ west build -p auto -b mps2_an521 \
  zephyr/samples/sensor/sensor_shell/ \
  -- -DCONFIG_I2C_SHELL=y

$ qemu-system-arm -M mps2-an521 -device loader,file=build/zephyr/zephyr.elf \
  -serial stdio \
  -monitor tcp:localhost:4444,server,nowait \
  -device lsm303dlhc_mag,id=lsm303,address=0x1E

uart:~$ i2c scan I2C_SHIELD1 
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:             -- -- -- -- -- -- -- -- -- -- -- -- 
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- 1e -- 
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
70: -- -- -- -- -- -- -- --                         
1 devices found on I2C_SHIELD1

Zephyr's LSM303DLHC driver can be enabled in a sample by adding the following
DTS overlay:

/*
 * Copyright (c) 2021 Linaro Limited
 *
 * SPDX-License-Identifier: Apache-2.0
 */

&i2c_shield1 {
        lsm303dlhc-magn@1e {
                compatible = "st,lsm303dlhc-magn";
                reg = <0x1e>;
                label = "LSM303DLHC-MAGN";
        };
};

And the following KConfig settings:

CONFIG_I2C=y
CONFIG_I2C_SHELL=y
CONFIG_SENSOR=y
CONFIG_LSM303DLHC_MAGN=y

When a sample with the above settings is run, the magnetometer can be read
via the shell sensor command:

uart:~$ sensor get LSM303DLHC-MAGN magn_xyz
channel idx=11 magn_xyz x =   0.000000 y =   0.000000 z =   0.000000

Set the y-axis (via human monitor or qmp) to 1100, which equals 1 Gauss
with the default gain settings:

(qemu) qom-set lsm303 mag_y 1100
qom-set lsm303 mag_y 1100

And test again in Zephyr:

uart:~$ sensor get LSM303DLHC-MAGN magn_xyz
channel idx=11 magn_xyz x =   0.000000 y =   1.000000 z =   0.000000