dt-bindings: mfd: ti,am3359-tscadc: Add a yaml description for this MFD

There is a very light description of this MFD in a text file dedicated
to a touchscreen controller (which is one of the two children of the
MFD). Here is now a complete yaml description.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20211015081506.933180-3-miquel.raynal@bootlin.com