arm: socfpga: clean up socfpga_common.h

Remove outdated macros and comments (not used any more, outdated due to
DM conversion) from socfpga_common.h.

Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Series-changes: 3
- changed commit message: s/defines/macros and comments/

Series-changes: 2
- remove even more outdated things