dt-bindings: Add 1027/1029/1031 SPI ADCs as trivial devices

In the same time, remove the dedicated bindings file which is now
useless.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>