siemens: cosmetic: remove unused and rename defines

For dxr2 board DXR2_IOCTRL_VAL is set by data in EEPROM. In pxm2
board it does not make sense to have dxr2 as prefix. Replace it with
more meaningful DDR prefix.

Signed-off-by: Samuel Egli <samuel.egli@siemens.com>
Cc: Pascal Bach <pascal.bach@siemens.com>
Cc: Roger Meier <r.meier@siemens.com>
Cc: Heiko Schocher <hs@denx.de>
Cc: Wolfgang Denk <wd@denx.de>