drivers: mbox: refactor API

This patch refactors the MBOX API, so that it aligns with some common
Zephyr principles, main changes include:

- There is now a non-DT specific API that takes a regular struct device
  and a channel identifier, instead of a struct that contains both. This
  API should be used for cases where DT is not an option.
- There is a "dt-spec" variant of the API, meant to be used from
  devicetree specifications.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>