ppc: Fix DBSR_IAx defines in <bedbug/regs.h>

While adding asm/cache.h to common.h for PPC targets, I got an
error about multiple definitions of some DBSR_ macros. While
scanning these defines, I noticed that some where defined not
correctly for all PPC variants. So I removed all unused defines,
and corrected the ones really used by bedbug (book-e vs. ppc40x).

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Wolfgang Denk <wd@denx.de>