cmd_cache: use cache/invalidate functions available in common.h

flush_dcache()/flush_icache() aren't defined in common.h,
flush_dcache_all()/invalidate_icache_all() however are.

Let the icache and dcache commands use those instead.

Signed-off-by: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
Acked-by: Mike Frysinger <vapier@gentoo.org>