refactor(imx8mn): cleanup the mmap region settings

No functional change.

Introduce the bl_regions array to gather all regions and make use of the
MAP_REGION_FLAT() macro. The array is than passed to mmap_add() to map
all regions. While on it introduce some defines so the addr, size and
flags can be read more easily.

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Change-Id: Id5849d2a7326a943927f458f1c6abbc041f5be18