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/mimxrt/modmachine.c

@@ -33,6 +33,7 @@
 #include "extmod/machine_pulse.h"
 #include "extmod/machine_signal.h"
 #include "extmod/machine_spi.h"
+#include "extmod/modmachine.h"
 #include "shared/runtime/pyexec.h"
 #include "led.h"
 #include "pin.h"
@@ -204,7 +205,9 @@ STATIC const mp_rom_map_elem_t machine_module_globals_table[] = {
     #endif
     { MP_ROM_QSTR(MP_QSTR_SPI),                 MP_ROM_PTR(&machine_spi_type) },
     { MP_ROM_QSTR(MP_QSTR_UART),                MP_ROM_PTR(&machine_uart_type) },
+    #if MICROPY_PY_MACHINE_WDT
     { MP_ROM_QSTR(MP_QSTR_WDT),                 MP_ROM_PTR(&machine_wdt_type) },
+    #endif
 
     { MP_ROM_QSTR(MP_QSTR_idle),                MP_ROM_PTR(&machine_idle_obj) },
     { MP_ROM_QSTR(MP_QSTR_deepsleep),           MP_ROM_PTR(&machine_deepsleep_obj) },