sh_eth: Add asm/cacheflush.h

Add include asm/cacheflush.h,  because declaration of __flush_purge_region
moved to asm/cacheflush.h.

Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Signed-off-by: David S. Miller <davem@davemloft.net>