Added API functions to read/write device/channel attributes by type

Those functions are:

iio_device_attr_read_bool
iio_device_attr_read_longlong
iio_device_attr_read_double

iio_device_attr_write_bool
iio_device_attr_write_longlong
iio_device_attr_write_double

iio_channel_attr_read_bool
iio_channel_attr_read_longlong
iio_channel_attr_read_double

iio_channel_attr_write_bool
iio_channel_attr_write_longlong
iio_channel_attr_write_double

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>