cpb: define the SPEAKER_ENABLE_PIN

This allows the board to disable the onboard speaker until explicitly
enabled in user code.

Testing performed on a CPB:
 * Touching the AUDIO pin with a fingertip no longer generates noise/buzz
 * Generating a waveform with `simpleio.tone` produces no sound by default
 * When the board.SPEAKER_ENABLE is configured as a digital output and
   set True, `simpleio.tone` does produce sound

Note that while guides should include information about SPEAKER_ENABLE, it's
possible that some users who omitted it could view this as a breaking change.
They can fix it by simply adding code similar to

    speaker_enable = digitalio.DigitalInOut(board.SPEAKER_ENABLE)
    speaker_enable.direction = digitalio.Direction.OUTPUT
    speaker_enable.value = True

before using the internal speaker.

Closes: #2258

ports/nrf/boards/circuitplayground_bluefruit/mpconfigboard.h

@@ -70,3 +70,5 @@
 
 #define DEFAULT_UART_BUS_RX         (&pin_P0_30)
 #define DEFAULT_UART_BUS_TX         (&pin_P0_14)
+
+#define SPEAKER_ENABLE_PIN          (&pin_P1_04)