iio: Rename iio/dds to iio/frequency

Generalize naming to allow other frequency synthesis techniques as well.
No functional changes.

Signed-off-by: Michael Hennerich <[e-mail]>
Acked-by: Jonathan Cameron <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/staging/iio/Kconfig

···
 source "drivers/staging/iio/addac/Kconfig"
 source "drivers/staging/iio/cdc/Kconfig"
 source "drivers/staging/iio/dac/Kconfig"
-source "drivers/staging/iio/dds/Kconfig"
+source "drivers/staging/iio/frequency/Kconfig"
 source "drivers/staging/iio/gyro/Kconfig"
 source "drivers/staging/iio/impedance-analyzer/Kconfig"
 source "drivers/staging/iio/imu/Kconfig"

drivers/staging/iio/Makefile

···
 obj-y += addac/
 obj-y += cdc/
 obj-y += dac/
-obj-y += dds/
+obj-y += frequency/
 obj-y += gyro/
 obj-y += impedance-analyzer/
 obj-y += imu/