dts: bindings: comparator: define silabs,acmp bindings

Defines bindings that are compatible with silabs acmp.
`input-positive` and `input-negative` are required
properties to be configured by an application.
It is recommended to use the bindings generated in
`include/zephyr/dt-bindings/comparator/silabs_acmp.h`
and reference your part's design book when configuring
values for these properties.

Signed-off-by: Christian Galante <christian.galante@silabs.com>