refactor(imx8mp): cleanup the mmap region settings

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. No functional change done.

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Change-Id: I7f637beea61138a86d691cd78fba2dd17e4dc925