iio: st_sensors: announce registered sensors

It is pretty helpful to know already from dmesg that a certain
device is successfully registered, instead of having to browse
sysfs to see if it's actually there.

Cc: Lee Jones <lee.jones@linaro.org>
Cc: Denis CIOCCA <denis.ciocca@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>