Adds Kconfig option CONFIG_DEVMEM that allows the
/dev/mem device to be disabled.

Option defaults to /dev/mem enabled.
Signed-off-by: NRob Ward <robert.ward114@googlemail.com>
Acked-by: NArnd Bergmann <arnd@arndb.de>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>