doc: updates concerning ENTROPY_GENRATOR auto-enabling

Update the migration-guide to inform the users that now
CONFIG_ENTROPY_GENERATOR is automatically enabled as soon as
devicetree property "zephyr,entropy" is set.

Update also the "General recommendations" section of the Board Porting
Guide on the same topic.

Signed-off-by: Valerio Setti <vsetti@baylibre.com>