esp32/machine_pin: Add new hold keyword argument and remove PULL_HOLD.

The current pull=Pin.PULL_HOLD argument doesn't make a lot of sense in the
context of what it actually does vs what the ESP32 quickref document says
it does.

This commit removes PULL_HOLD and adds a new hold=True|False keyword
argument to Pin()/Pin.init().  Setting this to True will cause the ESP32 to
lock the configuration of the pin – including direction, output value,
drive strength, pull-up/-down – such that it can't be accidentally changed
and will be retained through a watchdog or internal reset.

Fixes issue #8283, and see also #8284.

docs/esp32/quickref.rst

@@ -177,6 +177,14 @@ safe maximum source/sink currents and approximate internal driver resistances:
  - ``Pin.DRIVE_2``: 20mA / 30 ohm (default strength if not configured)
  - ``Pin.DRIVE_3``: 40mA / 15 ohm
 
+The ``hold=`` keyword argument to ``Pin()`` and ``Pin.init()`` will enable the
+ESP32 "pad hold" feature. When set to ``True``, the pin configuration
+(direction, pull resistors and output value) will be held and any further
+changes (including changing the output level) will not be applied. Setting
+``hold=False`` will immediately apply any outstanding pin configuration changes
+and release the pin. Using ``hold=True`` while a pin is already held will apply
+any configuration changes and then immediately reapply the hold.
+
 Notes:
 
 * Pins 1 and 3 are REPL UART TX and RX respectively
@@ -549,6 +557,10 @@ deep-sleep mode::
     pin.init(pull=None)
     machine.deepsleep(10000)
 
+Output-configured RTC pins will also retain their output direction and level in
+deep-sleep if pad hold is enabled with the ``hold=True`` argument to
+``Pin.init()``.
+
 Non-RTC GPIO pins will be disconnected by default on entering deep-sleep.
 
 SD card