hwmon: rpi: add module alias to raspberrypi-hwmon

hwmon: rpi: add module alias to raspberrypi-hwmon

The raspberrypi-hwmon driver doesn't automatically load, although it does work
when loaded, by adding the alias it auto loads as expected when built as a
module. Tested on RPi2/RPi3 on 32 bit kernel and RPi3B+ on aarch64 with
Fedora 28 and a patched 4.18 RC kernel.

Fixes: 3c493c885cf ("hwmon: Add support for RPi voltage sensor")
Signed-off-by: Peter Robinson <pbrobinson@gmail.com>
CC: Stefan Wahren <stefan.wahren@i2se.com>
CC: Eric Anholt <eric@anholt.net>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>