asm-generic/io.h: remove asm/cacheflush.h include

Including <asm/cacheflush.h> from <asm-generic/io.h> prevents
cacheflush.h being able to use I/O functions like readl and writel due
to circular include dependencies. It doesn't appear as if anything from
cacheflush.h is actually used by the generic io.h, so remove the
include.

I've compile tested a defconfig compilation of blackfin, openrisc (which
needed <asm/pgtable.h> including from it's <asm/io.h> to get the PAGE_*
definitions), and xtensa.

Other architectures which use asm-generic/io.h are score and unicore32,
and looking at their io.h I don't see any obvious problems.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Acked-by: Jonas Bonn <jonas@southpole.se>
Cc: Chris Zankel <chris@zankel.net>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Cc: Mike Frysinger <vapier@gentoo.org>
Cc: Chen Liqin <liqin.chen@sunplusct.com>
Cc: Lennox Wu <lennox.wu@gmail.com>
Cc: Guan Xuetao <gxt@mprc.pku.edu.cn>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>