siemens: add ddr full memory test

siemens: add ddr full memory test

Add siemens specific memory test. Enable it through Kconfig option
SPL_CMT. The test is required from our HW team. It runs over
temperature during many days:
* must run indefinitively through the *whole* DDR area,
  so we cannot use linux memtest for example.
* must write/read/check all values

Signed-off-by: Enrico Leto <enrico.leto@siemens.com>
Signed-off-by: Heiko Schocher <hs@denx.de>