Fixes: 65071bd arm: Clarify memory layout calculation

arm_mem_barebox_image() shall return the beginning of the barebox
image (and thus the end of the malloc region). For relocatable
images we can return a suitable location, but for non relocatable
images we do not have a choice: We must return TEXT_BASE. If TEXT_BASE
happens to be outside the memory region between membase and endmem
we can return the base of the ramoops area.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Markus Pargmann <mpa@pengutronix.de>