include: linux: iio: add IIO_ATTR_{RO, WO, RW} and IIO_DEVICE_ATTR_{RO, WO, RW} macros

Add new macros: IIO_ATTR_RO, IIO_ATTR_WO, IIO_ATTR_RW,
IIO_DEVICE_ATTR_RO, IIO_DEVICE_ATTR_WO and IIO_DEVICE_ATTR_RW to reduce
the amount of boiler plate code that is needed for creating new
attributes. This mimics the *_RO, *_WO, and *_RW macros that are found
in include/linux/device.h and include/linux/sysfs.h.

Signed-off-by: Brian Masney <masneyb@onstation.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>