iio: adc: ti_am335x_adc: Add the am437x compatible

This driver supports both the legacy controller (am33xx) and the
extended one (am437x), so let's add a new compatible.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20211015081506.933180-46-miquel.raynal@bootlin.com