modules: cmsis_nn: Move Kconfig prompt out of CMSIS-DSP menu

The `CMSIS_NN` Kconfig previously depended on the `CMSIS_DSP` Kconfig,
and this placed the "CMSIS-NN Library Support" prompt under the "CMSIS-
DSP Library Support" menu (note that CMSIS-DSP is a menuconfig).

Since the CMSIS-NN library, although it requires the CMSIS-DSP library,
is not a subordinate component of the CMSIS-DSP library, remove its
dependency on `CMSIS_DSP` and make it select the Kconfig symbol
instead.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>