devfs: factor out core devfs functionality ...

This makes it possible to compile without devfs. devfs_create/devfs_remove
is used by drivers and thus must still be present even without devfs support.
Also, this patch adds cdev_open/cdev_close/cdev_flush/cdev_ioctl calls to
work with devices without using the file api.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>