dt: bindings: i2c/trivial-devices.txt: Add 2 iio supported accelerometers

Add compatible strings for 2 accelerometers which follow the trivial
i2c device bindings and have been supported by the iio subsystem for
a while now.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>