nrf52840: allow 12 voices

In my testing, the feather bluefruit can do 12 voices at 48kHz!
This is a simple case (no LFOs, no ring modulation) so other situations might
require a lower sample rate, fewer voices, or both.

The sound is a little artifacty & low-fi but I think that's down to the speaker
& use of PWM instead of I2S.

ports/nrf/mpconfigport.mk

@@ -20,6 +20,7 @@ CIRCUITPY_AUDIOBUSIO ?= 1
 CIRCUITPY_AUDIOCORE ?= 1
 CIRCUITPY_AUDIOMIXER ?= 1
 CIRCUITPY_AUDIOPWMIO ?= 1
+CIRCUITPY_SYNTHIO_MAX_CHANNELS = 12
 
 # Native BLEIO is not compatible with HCI _bleio.
 CIRCUITPY_BLEIO_HCI = 0