ppc: Include <asm/cache.h> in common.h

This is needed for the patch "cache: add default setting for
CONFIG_SYS_CACHELINE_SIZE" from Anton Staaf. As cache.h defines
CONFIG_SYS_CACHELINE_SIZE for PPC targets.

This will remove the following warnings/errors:

include/common.h:819:2: warning: #warning CONFIG_SYS_CACHELINE_SIZE not defined, using __BIGGEST_ALIGNMENT__
cache.c:33: error: '__BIGGEST_ALIGNMENT__' undeclared (first use in this function)

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Anton Staaf <robotboy@chromium.org>
Cc: Wolfgang Denk <wd@denx.de>