Add i.MXRT documentation for compatible string.

Cc: Giulio Benetti <giulio.benetti@benettiengineering.com>
Signed-off-by: Jesse Taube <Mr.Bossman075@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211125211443.1150135-11-Mr.Bossman075@gmail.comSigned-off-by: Ulf Hansson <ulf.hansson@linaro.org>