Refactor i.MX6UL processor code

* Add Addr and size definition for most i.MX6UL devices in i.MX6UL 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: d579043fbd4e4b490370783fda43fc02c8e9be75.1692964892.git.jcd@tribudubois.net
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>