common: add memtest.c with mem_test routine ...

Add mem_test routine. Useful to detect timing problems if someone
porting a new device to barebox. This test includes a data bus test,
address bus test and integrity check of memory.

This mem_test routine has as parameter start and end address of testing
space. The last parameter can skip the integrity check.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>