arch: arm: cortex_a_r/cortex_m: fix text region overlap with ram in !XIP

ROM_ADDR and RAM_ADDR is same when CONFIG_XIP is not defined.
If we reset the location for RAMABLE region to RAM_ADDR text region,
which is part of rom, overlaps with initial ram region.
This commit tries to avoid this incorrect behaviour for cortex_a
and cortex_m

Signed-off-by: Simon Maurer <mail@maurer.systems>