hexagon: Fix build failures in linux-next

hexagon:defconfig fails to build in linux-next since commit 332fd7c4
("genirq: Generic chip: Change irq_reg_{readl,writel} arguments").

The primary build failure is

arch/hexagon/include/asm/cacheflush.h: In function 'copy_to_user_page':
arch/hexagon/include/asm/cacheflush.h:89:22: error: 'VM_EXEC' undeclared

This is the result of including of <linux/io.h> from <linux/irq.h>,
which is now necessary due to the use of readl and writel from irq.h.
This causes recursive inclusions in hexagon code; cacheflush.h is included
from mm.h prior to the definition of VM_EXEC.

Fix the problem by moving copy_to_user_page from the hexagon include file to
arch/hexagon/mm/cache.c, similar to other architectures. After this change,
several redefinitions of readl and writel are reported. Those are caused
by recursive inclusions of io.h and asm/cacheflush.h. Fix those problems by
reducing the number of files included from those files. Also, it was necessary
to stop including asm-generic/cacheflush.h from asm/cacheflush.h. Instead,
functionality originally provided by asm-generic/cacheflush.h is now coded
in asm/cacheflush.h directly.

Cc: Kevin Cernekee <cernekee@gmail.com>
Cc: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
signed-off-by: Richard Kuo <rkuo@codeaurora.org>