Code for calculating total amount of memory found in
imx6_mmdc_add_mem(), imx6q_barebox_entry() and imx6ul_barebox_entry()
is exaclty the same, so refactor the code to avoid duplication.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>