Refactor i.MX7 processor code

Refactor i.MX7 processor code

* Add Addr and size definition for all i.MX7 devices in i.MX7 header file.
* Use those newly defined named constants whenever possible.
* Standardize the way we init a familly of unimplemented devices
  - SAI
  - PWM
  - CAN
* Add/rework few comments

Signed-off-by: Jean-Christophe Dubois <jcd@tribudubois.net>
Message-id: 59e195d33e4d486a8d131392acd46633c8c10ed7.1692964892.git.jcd@tribudubois.net
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>