refactor(imx8mn): make use of setup_page_tables()

No functional change.

Use the setup_page_tables() helper function which does the three calls
for us. Also the function has some logging support which will be nice
during debugging.

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Change-Id: I2f0182f19300a3a57bbeb7e2107c5fb5525dd0c1