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>

ports/esp32/esp32_common.cmake

@@ -53,6 +53,7 @@ list(APPEND MICROPY_SOURCE_DRIVERS
 )
 
 list(APPEND MICROPY_SOURCE_PORT
+    adc.c
     main.c
     ppp_set_auth.c
     uart.c
@@ -66,7 +67,6 @@ list(APPEND MICROPY_SOURCE_PORT
     machine_timer.c
     machine_pin.c
     machine_touchpad.c
-    machine_adcblock.c
     machine_dac.c
     machine_i2c.c
     modmachine.c