arch/powerpc/lib/board.c: make (mostly) checkpatch-clean

CodingStyle cleanup.

Accepted (in this cleanup) checkpatch messages:

- externs should be avoided
  (to be cleaned up later)
- no spaces at the start of a line
  (accepted in multi-line #if's)
- Macros with complex values
  (false reports)
- do not use assignment in if condition
  (accepted in one place, where avoiding it would have required an
  additional level of nesting, resulting in less readable code)
Signed-off-by: NWolfgang Denk <wd@denx.de>