extmod/machine_adc_block: Factor esp32 ADCBlock bindings to common code.

This is a code factoring to have the Python bindings in one location, and
all the ports use those same bindings.  At this stage only esp32 implements
this class, so the code for the bindings comes from that port.

The documentation is also updated to reflect the esp32's behaviour of
ADCBlock.connect().

Signed-off-by: Damien George <damien@micropython.org>
diff --git a/ports/esp32/mpconfigport.h b/ports/esp32/mpconfigport.h
index b4ec082..706baae 100644
--- a/ports/esp32/mpconfigport.h
+++ b/ports/esp32/mpconfigport.h
@@ -99,10 +99,11 @@
 #define MICROPY_PY_MACHINE_ADC              (1)
 #define MICROPY_PY_MACHINE_ADC_INCLUDEFILE  "ports/esp32/machine_adc.c"
 #define MICROPY_PY_MACHINE_ADC_ATTEN_WIDTH  (1)
-#define MICROPY_PY_MACHINE_ADC_BLOCK        (1)
 #define MICROPY_PY_MACHINE_ADC_INIT         (1)
 #define MICROPY_PY_MACHINE_ADC_READ         (1)
 #define MICROPY_PY_MACHINE_ADC_READ_UV      (1)
+#define MICROPY_PY_MACHINE_ADC_BLOCK        (1)
+#define MICROPY_PY_MACHINE_ADC_BLOCK_INCLUDEFILE "ports/esp32/machine_adc_block.c"
 #define MICROPY_PY_MACHINE_PIN_MAKE_NEW     mp_pin_make_new
 #define MICROPY_PY_MACHINE_BITSTREAM        (1)
 #define MICROPY_PY_MACHINE_PULSE            (1)