DSP: Fix stale reference to `CONFIG_CMSIS_DSP_MVEF`

This commit fixes the stale references to the `CONFIG_CMSIS_DSP_MVEF`
and `CONFIG_CMSIS_DSP_HELIUM` Kconfig symbols, which were replaced with
the arch-level symbol `CONFIG_ARMV8_1_M_MVEF`.

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