extmod/machine_wdt: Factor ports' WDT Python bindings to common code.

There are currently 7 ports that implement machine.WDT and a lot of code is
duplicated across these implementations.  This commit factors the common
parts of all these implementations to a single location in
extmod/machine_wdt.c.  This common code provides the top-level Python
bindings (class and method wrappers), and then each port implements the
back end specific to that port.

With this refactor the ports remain functionally the same except for:

- The esp8266 WDT constructor now takes keyword arguments, and accepts the
  "timeout" argument but raises an exception if it's not the default value
  (this port doesn't support changing the timeout).

- The mimxrt and samd ports now interpret the argument to WDT.timeout_ms()
  as signed and if it's negative truncate it to the minimum timeout (rather
  than it being unsigned and a negative value truncating to the maximum
  timeout).

Signed-off-by: Damien George <damien@micropython.org>

ports/rp2/CMakeLists.txt

@@ -126,7 +126,6 @@ set(MICROPY_SOURCE_PORT
     machine_spi.c
     machine_timer.c
     machine_uart.c
-    machine_wdt.c
     main.c
     modmachine.c
     modrp2.c