We hard code memory size at three places:
 - In the configuration we use to initialize the DDRAM controller
 - In the minimal available size passed from PBL to barebox proper
 - In the device tree memory node override

Remove the two latter ones and replace them with code that reads the
size back from controller.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>